Calling Send() with large objects errors out in the spdy implementation

[BrianBland]

The spdy channel.Send method will return a spdy.InvalidDataFrame error if the serialized size of the message is greater than 16MB (the spdy max data frame size). This could be fixed with a combination of automatic data frame chunking (moby/spdystream#31) and using the underlying spdystream object as an io.Reader for msgpack deserialization on the Receive side.