fix: throw on missing refresh token in all cases

[JustinBeckwith]

Fixes #606. In most cases, this library would throw if an attempt to refresh a token was made, and no refresh token was available. It turns out there was a loophole here if the getRequestHeaders method was called, and the throw would get lost in the shuffle. Instead, a call to the OAuth2 refresh endpoint would get made with an empty refresh token, leading to a less awesome exception. This change:

• moves the place where we throw
• adds a test to verify the behavior
• Adds a describe block in the test file

Make sure to hide whitespace before reviewing this one :)

[codecov]

Codecov Report

Merging #670 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #670   +/-   ##
=======================================
  Coverage   88.29%   88.29%           
=======================================
  Files          18       18           
  Lines         769      769           
  Branches       83       83           
=======================================
  Hits          679      679           
  Misses         79       79           
  Partials       11       11

Impacted Files	Coverage Δ
src/auth/oauth2client.ts	86.45% <100%> (ø)	⬆️

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[codecov]

Codecov Report

Merging #670 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #670   +/-   ##
=======================================
  Coverage   88.29%   88.29%           
=======================================
  Files          18       18           
  Lines         769      769           
  Branches       83       83           
=======================================
  Hits          679      679           
  Misses         79       79           
  Partials       11       11

Impacted Files	Coverage Δ
src/auth/oauth2client.ts	86.45% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8cc5522...2637c7e. Read the comment docs.

[alexander-fenster]

Rubber stamp even though I don't fully understand how it works :(