Make the db and statemachine default is the same

**What does this MR do?**
Remove a warning when you run the test suite:
`Both MergeRequestDiff and its :state machine have defined a different default for "state". Use only one or the other for defining defaults to avoid unexpected behaviors.`

**Are there points in the code the reviewer needs to double check?**
Maybe there was a good reason to have them different. So please wait for travis to finish its build

**Why was this MR needed?**
To remove an annoying warning.