Fix issue where milestones API did not accept "all".

See GitHub API reference
  https://developer.github.com/v3/issues/milestones/#parameters

It is already documented that it is supported in Milestones#list
  https://github.com/peter-murach/github/blob/c71f4fb8f162e0d6bc12bc8ef0aa190f953ad65e/lib/github_api/client/issues/milestones.rb#L23

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I wasn't sure if/how/where to add specs.  I don't see options tested anywhere else, and I didn't know if there was a pattern I should be following.