RFR: SKARA-1333: Sponsor label only removed if PR is actually sponsored

[Guoxiong Li]

Hi all,

If an author types `/integrate` command, the `sponsor` label is added. When the author becomes the committer and types the command `/integrate` again, the `sponsor` label doesn't been removed. It is good to remove the label `sponsor`, too, just like the labels `rfr` and `ready`. This patch fixes it and adds a test case.

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - SKARA-1333: Sponsor label only removed if PR is actually sponsored

Changes: https://git.openjdk.java.net/skara/pull/1282/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1282&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1333
  Stats: 100 lines in 3 files changed: 96 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/1282.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1282/head:pull/1282

PR: https://git.openjdk.java.net/skara/pull/1282