RFR: 1588: Bridge messages should not be sent for PRs in draft state

[Guoxiong Li]

Hi all,

This patch prevents the `mailling bridge bot` from sending emails when a PR is draft.
And the related test case is added.

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - SKARA-1588

Changes: https://git.openjdk.org/skara/pull/1469/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1469&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1588
  Stats: 124 lines in 3 files changed: 121 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/skara/pull/1469.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1469/head:pull/1469

PR: https://git.openjdk.org/skara/pull/1469