Commit related to issue #61 - Allowing reset fields except in default mandatory ones

This pull request assures backward compatibility is not broken. The way I choose to implement it maybe is not so straightforward but I didn't want to change so much the architecture code. I've added a new method in RedmineManager called resetIssue for the goal of this issue meanwhile update method will do the same as before.

Well, not exactly the same as I found that the code was allowing reset start_date. I've assumed that is a bug so I've changed it. That's why I had to update priorityIdIsAddedToXMLIfProvided test.