Set Accept header to reasonable default

[bitwiseman]

Description

The Accept header should default to a reasonable default. This also makes the test resources for OkHttpClient and HttpURLConnection the same, allowing for better test coverage.

Before submitting a PR:

We love getting PRs, but we hate asking people for the same basic changes every time.

• Push your changes to a branch other than main. Create your PR from that branch.
• Add JavaDocs and other comments
• Write tests that run and pass in CI. See CONTRIBUTING.md for details on how to capture snapshot data.
• Run mvn -D enable-ci clean install site locally. If this command doesn't succeed, your change will not pass CI.

When creating a PR:

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• Enable "Allow edits from maintainers".

[codecov]

Codecov Report

Merging #1289 (260ae86) into main (eef3c58) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main    #1289   +/-   ##
=========================================
  Coverage     77.50%   77.50%           
- Complexity     1905     1906    +1     
=========================================
  Files           189      189           
  Lines          6000     6002    +2     
  Branches        328      329    +1     
=========================================
+ Hits           4650     4652    +2     
  Misses         1150     1150           
  Partials        200      200           

Impacted Files	Coverage Δ
src/main/java/org/kohsuke/github/GitHubClient.java	89.47% <100.00%> (+0.10%)	⬆️

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