actually fix regression with updating issue labels

[shiftkey]

Fixes #715

When you update an issue, the value in Labels is important. From the docs:

Labels to associate with this issue. Pass one or more Labels to replace the set of Labels on this Issue. Send an empty array ([ ]) to clear all Labels from the Issue.

There's three scenarios we need to support here, and I think this is the best compromise:

• you start with .ToUpdate() to setup your update request - this gives you a null collection of branches, which means you're not changing anything server-side.
• calling AddLabel manages the internal state of the list - and give you a collection which will replace whatever is currently there
• calling ClearLabels should give you an empty collection - if this is used, it'll drop whatever labels you have defined on the server.
• write some docs

How does that feel @thedillonb @haacked?

[thedillonb]

@shiftkey, sounds good to me!

[shiftkey]

Ready for review

[haacked]

Nice!