ReturnsLazily of task #256
Improved test name
Made Returns of Task to leverage new ReturnsLazily of task
added Task overload of ReturnsNextFromSequence
Added support for Returning Task<T> to ReturnsLazily overloads that take parameters
Created TaskHelper to reduce code duplication.
Updated tests to use FluentAssertions.
Added missing AssertThatValueProducerSignatureSatisfiesCallSignature to Task-based ReturnsLazily with 1 argument.
Fixed incorrect comment about ReturnsNextFromSequence returning null when all values in the sequence have been returned.