…marshal for addcomment method on issue service

What type of PR is this?

• bug

What this PR does / why we need it:

We have experienced issues using the AddCommentWithContext method with the following input:

jid := "ABC-123"
c := &jira.Comment{
  Body: "Some comment body",
}

cmntBytes, _ := json.Marshal(c)

cmnt, resp, err := jc.Issue.AddComment(context.Background(), jid, c)

I receive

Invalid request payload. Refer to the REST API documentation and try again.: request failed. Please analyze the request body for more details. Status code: 400

This likely happens because the marshalled output from json.Marshal comes up as

{
  "author": {
    "avatarUrls": {},
    "groups": {},
    "applicationRoles": {}
  }
  "body": "Some comment body",
  "updateAuthor": {
    "avatarUrls": {},
    "groups": {},
    "applicationRoles": {}
  },
  "visibility": {}
}

Which issue(s) this PR fixes:

Fixes #671

Special notes for your reviewer:

Additional documentation e.g., usage docs, etc.: