Fix just headers not being applied in dev mode

[ijjk]

While responding to #11752 I noticed that headers aren't applied in dev mode when you only have headers and no rewrites or redirects. This fixes that and adds test cases to ensure each custom route mode works individually

[ijjk]

Stats from current PR

Default Server Mode

General Overall increase ⚠️

	zeit/next.js canary	ijjk/next.js fix/dev-only-headers	Change
buildDuration	11.3s	11.2s	-57ms
nodeModulesSize	52.5 MB	52.5 MB	⚠️ +51 B

Client Bundles (main, webpack, commons)

	zeit/next.js canary	ijjk/next.js fix/dev-only-headers	Change
main-HASH.js gzip	6.25 kB	6.25 kB	✓
webpack-HASH.js gzip	746 B	746 B	✓
de003c3a9d30..e3d8.js gzip	10.2 kB	10.2 kB	✓
framework.HASH.js gzip	39.1 kB	39.1 kB	✓
Overall change	56.3 kB	56.3 kB	✓

Client Bundles (main, webpack, commons) Modern

	zeit/next.js canary	ijjk/next.js fix/dev-only-headers	Change
main-HASH.module.js gzip	4.78 kB	4.78 kB	✓
webpack-HASH..dule.js gzip	746 B	746 B	✓
de003c3a9d30..dule.js gzip	6.78 kB	6.78 kB	✓
framework.HA..dule.js gzip	39.1 kB	39.1 kB	✓
Overall change	51.5 kB	51.5 kB	✓

Legacy Client Bundles (polyfills)

	zeit/next.js canary	ijjk/next.js fix/dev-only-headers	Change
polyfills-HASH.js gzip	26.3 kB	26.3 kB	✓
Overall change	26.3 kB	26.3 kB	✓

Client Pages

	zeit/next.js canary	ijjk/next.js fix/dev-only-headers	Change
_app.js gzip	1.24 kB	1.24 kB	✓
_error.js gzip	3.15 kB	3.15 kB	✓
hooks.js gzip	664 B	664 B	✓
index.js gzip	222 B	222 B	✓
link.js gzip	2.03 kB	2.03 kB	✓
routerDirect.js gzip	279 B	279 B	✓
withRouter.js gzip	278 B	278 B	✓
Overall change	7.86 kB	7.86 kB	✓

Client Pages Modern

	zeit/next.js canary	ijjk/next.js fix/dev-only-headers	Change
_app.module.js gzip	594 B	594 B	✓
_error.module.js gzip	2.08 kB	2.08 kB	✓
hooks.module.js gzip	370 B	370 B	✓
index.module.js gzip	212 B	212 B	✓
link.module.js gzip	1.48 kB	1.48 kB	✓
routerDirect..dule.js gzip	271 B	271 B	✓
withRouter.m..dule.js gzip	270 B	270 B	✓
Overall change	5.28 kB	5.28 kB	✓

Client Build Manifests

	zeit/next.js canary	ijjk/next.js fix/dev-only-headers	Change
_buildManifest.js gzip	61 B	61 B	✓
_buildManife..dule.js gzip	61 B	61 B	✓
Overall change	122 B	122 B	✓

Rendered Page Sizes

	zeit/next.js canary	ijjk/next.js fix/dev-only-headers	Change
index.html gzip	918 B	918 B	✓
link.html gzip	926 B	926 B	✓
withRouter.html gzip	913 B	913 B	✓
Overall change	2.76 kB	2.76 kB	✓

---

Serverless Mode (Decrease detected ✓)

General Overall increase ⚠️

	zeit/next.js canary	ijjk/next.js fix/dev-only-headers	Change
buildDuration	11.3s	11.4s	⚠️ +95ms
nodeModulesSize	52.5 MB	52.5 MB	⚠️ +51 B

Client Bundles (main, webpack, commons)

	zeit/next.js canary	ijjk/next.js fix/dev-only-headers	Change
main-HASH.js gzip	6.25 kB	6.25 kB	✓
webpack-HASH.js gzip	746 B	746 B	✓
de003c3a9d30..e3d8.js gzip	10.2 kB	10.2 kB	✓
framework.HASH.js gzip	39.1 kB	39.1 kB	✓
Overall change	56.3 kB	56.3 kB	✓

Client Bundles (main, webpack, commons) Modern

	zeit/next.js canary	ijjk/next.js fix/dev-only-headers	Change
main-HASH.module.js gzip	4.78 kB	4.78 kB	✓
webpack-HASH..dule.js gzip	746 B	746 B	✓
de003c3a9d30..dule.js gzip	6.78 kB	6.78 kB	✓
framework.HA..dule.js gzip	39.1 kB	39.1 kB	✓
Overall change	51.5 kB	51.5 kB	✓

Legacy Client Bundles (polyfills)

	zeit/next.js canary	ijjk/next.js fix/dev-only-headers	Change
polyfills-HASH.js gzip	26.3 kB	26.3 kB	✓
Overall change	26.3 kB	26.3 kB	✓

Client Pages

	zeit/next.js canary	ijjk/next.js fix/dev-only-headers	Change
_app.js gzip	1.24 kB	1.24 kB	✓
_error.js gzip	3.15 kB	3.15 kB	✓
hooks.js gzip	664 B	664 B	✓
index.js gzip	222 B	222 B	✓
link.js gzip	2.03 kB	2.03 kB	✓
routerDirect.js gzip	279 B	279 B	✓
withRouter.js gzip	278 B	278 B	✓
Overall change	7.86 kB	7.86 kB	✓

Client Pages Modern

	zeit/next.js canary	ijjk/next.js fix/dev-only-headers	Change
_app.module.js gzip	594 B	594 B	✓
_error.module.js gzip	2.08 kB	2.08 kB	✓
hooks.module.js gzip	370 B	370 B	✓
index.module.js gzip	212 B	212 B	✓
link.module.js gzip	1.48 kB	1.48 kB	✓
routerDirect..dule.js gzip	271 B	271 B	✓
withRouter.m..dule.js gzip	270 B	270 B	✓
Overall change	5.28 kB	5.28 kB	✓

Client Build Manifests

	zeit/next.js canary	ijjk/next.js fix/dev-only-headers	Change
_buildManifest.js gzip	61 B	61 B	✓
_buildManife..dule.js gzip	61 B	61 B	✓
Overall change	122 B	122 B	✓

Serverless bundles Overall decrease ✓

	zeit/next.js canary	ijjk/next.js fix/dev-only-headers	Change
_error.js gzip	233 kB	233 kB	⚠️ +313 B
404.html gzip	1.32 kB	1.32 kB	✓
hooks.html gzip	956 B	956 B	✓
index.js gzip	233 kB	233 kB	-497 B
link.js gzip	243 kB	243 kB	⚠️ +76 B
routerDirect.js gzip	241 kB	241 kB	⚠️ +16 B
withRouter.js gzip	241 kB	241 kB	-16 B
Overall change	1.19 MB	1.19 MB	-108 B