fix: preserve default headers with custom headers

[pi0]

There is a breaking change with axios axios/axios@v0.19.2...v0.20.0 that when passing headers config when calling axios, it discards defaults.headers.common (used for both config and setHeader utility.

resolves #411

Update: Axios regression found by @1ed regarding defaults overriding: axios/axios#2844 <3

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Test flow for #411:

• /api/cookie endpoint that simply returns value of mycookie header if set
• Make a random string in client-side and set cookie "mycookie"
• Make SSR request
  • SSR request makes a call to /api/cookie in asyncData using $axios to check what headers are passed to API when using axios in server side
• Value is checked against initial random one (and passing)

Locally trying:

• Clone repo and run yarn install
• Switch to fix/411 branch
• Run yarn dev
• Go to http://localhost:3000/cookie

[codecov]

Codecov Report

Merging #452 (97c477f) into master (bec998c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #452   +/-   ##
=======================================
  Coverage   95.55%   95.55%           
=======================================
  Files           1        1           
  Lines          45       45           
  Branches       25       25           
=======================================
  Hits           43       43           
  Misses          2        2           

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