#394 adds missing header to POST requests

[spencerkordecki]

Description

Fixes #394 by adding the header of "Content-Type: application/x-www-form-urlencoded" which was automatically added by the usage of URLSearchParams(), but when that was removed in #373, the header was missing and caused an error with POST requests.

Related Issues

Fixes #394 , consequence of #373

Checklist:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you linted your code locally prior to submission?
• Have you successfully ran tests with your changes locally?

[codecov]

Codecov Report

Merging #395 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #395      +/-   ##
==========================================
+ Coverage   95.70%   95.74%   +0.03%     
==========================================
  Files          23       23              
  Lines         233      235       +2     
  Branches       21       21              
==========================================
+ Hits          223      225       +2     
  Misses         10       10              

Impacted Files	Coverage Δ
src/util.js	69.56% <100.00%> (+2.89%)	⬆️

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