support streaming HTTP body

[starius]

Streaming. If you use use_as_body:"true", you can also set
is_stream:"true". In this case the field must be of type io.ReadCloser.
On the client side put any object implementing io.ReadCloser to such
a field in Request. It will be read and closed by the library and used
as HTTP request body. On the server side your handler
should read from the reader passed in that field of Request.
(You don't have to read the entire body and to close it.)
For Response, on the server side, the handler must put any object
implementing io.ReadCloser to such a field of Response.
The library will use it to generate HTTP response's body and close it.
On the client side your code must read from that reader the entire response
and then close it.

[zmitry]

looks good