Add failing test from #723
Copy the input argument to +merge:

If it's an NSEnumerator, it'll become exhausted after the first
enumeration, invaliding future subscriptions.
Merge pull request #724 from ReactiveCocoa/merge-fast-enumeration-bug

Copy <NSFastEnumeration> argument to +merge:
Merge remote-tracking branch 'origin/master' into 2.0-development