Fixed System.Net.ProtocolViolationException issue cause of missed the ContentLength property

When I use RestRequest.AddFile (string name, string path) to add a file entity to upload in Unity3D which has a mono runtime inside, I found upload always failed with this exception:
>System.Net.ProtocolViolationException: The number of bytes to be written is greater than the specified ContentLength.
  at System.Net.WebConnectionStream.CheckWriteOverflow (Int64 contentLength, Int64 totalWritten, Int64 size) [0x00000] in <filename unknown>:0 
  at System.Net.WebConnectionStream.BeginWrite (System.Byte[] buffer, Int32 offset, Int32 size, System.AsyncCallback cb, System.Object state) [0x00000] in <filename unknown>:0 
  at System.Net.WebConnectionStream.Write (System.Byte[] buffer, Int32 offset, Int32 size) [0x00000] in <filename unknown>:0 
  at RestSharp.Extensions.MiscExtensions.CopyTo (System.IO.Stream input, System.IO.Stream output) [0x0001f] in /Users/helihua/unity_workspace/GowOnline/Assets/Library/RestSharp/Extensions/MiscExtensions.cs:72 
  at RestSharp.RestRequest+<AddFile>c__AnonStorey5E.<>m__3D (System.IO.Stream s) [0x0000c] in /Users/helihua/unity_workspace/GowOnline/Assets/Library/RestSharp/RestRequest.cs:128