Hold onto NSData so it doesn't get garbage-collected. Move stream initialization to property.
This fixes http://github.com/xamarin/Xamarin.Social/issues/23.

Licensed under MIT/X11
Dispose of NSData when disposing is true
Merge pull request #24 from stampsy/xamarin-pr-fix-crash-in-foundationresponse

Prevent NSData from getting GC'd in FoundationResponse