modify ReplaySubject (with specified capacity) to comply with original reactive extensions behaviour

modify ReplaySubject (with specified capacity) to comply with original reactive extensions behaviour

the capacity of ReplaySubject specifies maximum element count of the replay buffer

code:
https://rx.codeplex.com/SourceControl/latest#Rx.NET/Source/System.Reactive.Linq/Reactive/Subjects/ReplaySubject.cs
test:
https://rx.codeplex.com/SourceControl/latest#Rx.NET/Source/Tests.System.Reactive/Tests/Linq/Subjects/ReplaySubjectTest.cs