Fixed incorrect signature in TaskAwaiter<T>.GetResult and test modified.

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extra : source : c72badf7cb727b15b0fb45db82061862e3008687
extra : amend_source : 76e9581c8bd856c579012a69ee6efc6e93b84e7c
Merge pull request #545 from iskiselev/TaskAwaiterFix

Fixed incorrect signature in TaskAwaiter<T>.GetResult and test modified.