Request body buffering is not transport-specific.

Previously we were only buffering the request body when the transport
was HTTP. But we should buffer it for both SPDY and HTTP transports.

Closes https://github.com/square/okhttp/issues/1132

Also recover gracefully when retrying an empty POST body. This
is a related bug.

Closes https://github.com/square/okhttp/issues/1131
Merge pull request #1143 from square/jwilson_1110_emptysinnk

Request body buffering is not transport-specific.