Adds a new updateMergeRequest method with more attributes

[armsnyder]

I deprecated the existing updateMergeRequest method and added a new updateMergeRequest method which accepts more parameters. These new parameters correspond to the attributes listed here that were previously not settable.

Closes #205

Since there were now 3 different updateMergeRequest methods which contained duplicated code, I abstracted the common logic to a protected method

protected MergeRequest updateMergeRequest(Integer projectId, Integer mergeRequestIid, Form formData)

The reason it is protected and not private is so that in the future, users of this library can extend the class to add new attributes themselves, instead of waiting for the library to update. I'm curious if making it a public method would be preferable.

[gmessner]

@armsnyder
Thanks for the contribution, looks good and I always appreciate unut tests being included.