Edit issue not respect documentation

[mrbatista]

The rest api documentation allow use of update and fields properties on body request but the edit_issue apply data only to update property.
The update_issue method apply data only to fields property.

If api support request with both properties why use only one field at time?
At the moment I have overwrite method with my custom implementation.

[gonchik]

@mrbatista Feel free to send a PR :)

Happy to merge it