New extension methods for Reactive{Collection,Table}ViewSources.
Do not try to automagically convert strings to NSStrings.

IMHO this is the morally correct action to take.  iOS is going
to compare strings by reference, so:

  - Original code by @paulcbetts used a cast from string to
    NSString.  I have no idea how it works, but it does seem hackish.

  - I had modified the code to use new NSString(), which in
    hindsight obviously will not work.

  - This new version does not autoconvert anything at all, thus
    the process is transparent and does not rely on internal details.