Fix Rails/UniqBeforePluck to ignore uniq with block

[tejasbubane]

Before submitting a PR make sure the following are checked:

• Wrote good commit messages.
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it)
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.

Issue #3122
Also moved around some code in specs to use shared_context for ignored cases
Might conflict with #3124

[rrosenblum]

The code is going to need to be rebased. Aside from that, it looks good to me.

[tejasbubane]

@rrosenblum Rebased. There was a conflict in CHANGELOG. Not sure why the build did not get triggered this time. cc:/ @bbatsov