restsharp 105 & .Net 4.5.1 & some fixes and improvements

Updated to RestSharp 105 & fixed upgrade issue
Set target .Net framework to 4.5.1 & switched on warnings as errors
Fixed logic for UploadRequest.AllBytesWritten flag (now it's independent from the File & File.Data that was destroyed by BinaryContent disposer)
BinaryContent disposer prevent from external stream dispose