fix: do not override content-type if its given

[alexander-fenster]

Fixes #156 by forcing the Content-Type header only it's not set in options. It should work then because the header is set in https://github.com/googleapis/nodejs-googleapis-common/blob/8734154a219c3a2bf865dc830bc95fbb5a640794/src/apirequest.ts#L210 and should be just passed down all the way.

[bcoe]

👍 looks reasonable to me; imagine we'll want to be careful when rolling this out to libraries, but seems like the right change.

[codecov]

Codecov Report

Merging #158 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #158      +/-   ##
==========================================
+ Coverage   96.55%   96.58%   +0.03%     
==========================================
  Files           4        4              
  Lines         580      586       +6     
  Branches       98      100       +2     
==========================================
+ Hits          560      566       +6     
  Misses         19       19              
  Partials        1        1

Impacted Files	Coverage Δ
src/gaxios.ts	93.48% <100%> (+0.15%)	⬆️

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[alexander-fenster]

Let's wait for @JustinBeckwith to release this, just to have one more pair of 👀 checking the logic.