Add API to get a single issue

[keiichiw]

This PR adds the API to get a sigle issue.

Usage

$ curl http://localhost:8080/api/v3/repos/:owner/:repo/issues/:issue_number

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The second commit is a small fix of #1328.
This is independent from the first commit, but it's so small that I didn't divide into another PR.

Before submitting a pull-request to Gitbucket I have first:

• read the contribution guidelines
• rebased my branch over master
• verified that project is compiling
• verified that tests are passing
• squashed my commits as appropriate (keep several commits if it is relevant to understand the PR)
• marked as closed all issue ID that this PR should correct

[takezoe]

@kw-udon Great thanks! Could you update the supported API list as well?

[keiichiw]

@takezoe I updated the list.

[takezoe]

@kw-udon Thanks.