Overload the #update_issue method to allow partial updates

The old argument structure still works as usual:

    update_issue(repo, number, title, body, options)

But now, you have the option of passing the title and body
attributes in the options hash (or omitting these attributes
if they should not be changed):

    update_issue(repo, number, options)
Merge pull request #531 from codetree/allow-partial-issue-updates

Overload the #update_issue method to allow partial updates