add fabricators
add controller spec for star controller

@KitaitiMakoto

i told you i don wanna use controller spec,
but in this case, we need to use controller spec instead of request spec.
do you agree?
[REFACTOR] use find_or_create_by_user_id_and_message_id
[REFACTOR] use ActiveRecord::Base#increment

refs http://apidock.com/rails/ActiveRecord/Base/increment
[REFACTOR] adapt to transaction
return valid error response
add transaction spec
fix encoding

refs https://travis-ci.org/iiirc/iiirc/jobs/7209195
tiny fix for English [ci skip]
Alert on error
Revert "add fabricators"

This reverts commit 6a02e639d2155c84d502d17cd0baa7dd5a831b98.

Conflicts:
	spec/fabricators/message_fabricator.rb

refs:
https://github.com/iiirc/iiirc/commit/6a02e639d2155c84d502d17cd0baa7dd5a831b98#commitcomment-3233441
Add a fabricator for Message
Add a fabricator for Star
change spec according to a643eff7
Merge pull request #52 from iiirc/improvement-the-coverage-rate

Improvement the coverage rate