IssueLabelApp: Rewrite javadoc in English

주어진 한국어와 영어 문장이 많이 달라서, 일단, 주어진 영어를 수정했습니다.

When a new label is added, respond with 201 Created in the response body that includes a hash consisting of @link IssueLabel#id}, {@link IssueLabel#name}, {@link IssueLabel#color}, {@link IssueLabel#category} in {@code application/json}. But if the client cannot accept {@code application/json}, return 201 Created with an empty body.