ComposeMessageRequest tests

Added the tests of ComposeMessageRequest.

Closes #13
ReadMessagesRequest test

Added the tests of ReadMessageRequest.

Closes #14
Subscribe/unsubscribe tests

Added the tests of SubscribeRequest.

Closes #15
SearchRequest tests

Added the tests of SearchRequest.

Closes #16
Subreddit request test

Added the tests of SubredditRequest.

Closes #17
GetCommentRequest tests

Added the tests of GetCommentRequest.

Closes #18