RFR: 1695: Set GitHub API date version header

[Zhao Song]

In this patch, added new Header `X-GitHub-Api-Version` to all GitHub API calls.

-------------

Commit messages:
 - SKARA-1695

Changes: https://git.openjdk.org/skara/pull/1440/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1440&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1695
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1440.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1440/head:pull/1440

PR: https://git.openjdk.org/skara/pull/1440