middleware/proxy: favor __send__ for method dispatch

middleware/proxy: favor __send__ for method dispatch

"send" is more likely to be overridden in subclasses whereas
the Ruby runtime (at least 1.9.3) will warn loudly if any user
code (re)defines the "__send__" method.

For example, BasicSocket#send and UDPSocket#send in the Ruby
stdlib are wrappers for the send(2)/sendto(2) system calls,
and it's entirely possible an application could return a
Socket-subclass as a Rack response body.