Integrated: forge: add Accept HTTP headers for GitHub search API

[Erik Helin]

Hi all,

please review this patch that adds the appropriate HTTP headers for using GitHub's search REST endpoint. I also that the `RestRequest`s created in different constructors specify the same `Accept` headers.

Thanks,
Erik

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Commit messages:
 - forge: add Accept HTTP headers for GitHub search API

Changes: https://git.openjdk.java.net/skara/pull/900/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=900&range=00
  Stats: 8 lines in 1 file changed: 6 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/900.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/900/head:pull/900

PR: https://git.openjdk.java.net/skara/pull/900