Fix NSDocumentController.SharedDocumentController return type

[CallumDev]

Issue by dlech
Thursday May 29, 2014 at 21:24 GMT
Originally opened as mono#126

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See http://forums.xamarin.com/discussion/17120/why-is-nsdocumentcontroller-shareddocumentcontroller-exposed-as-an-nsobject

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dlech included the following code: https://github.com/mono/monomac/pull/126/commits