Prevent pushing a stream into both pending_send + pending_open (#235)

Prevent pushing a stream into both pending_send + pending_open,
Clear out variables from buffered streams that get a reset, and
ignore them when traversing the pending_send queue if
they are is_reset(). Add asserts that a stream cannot be in
pending_open & pending_send at the same time.

src/proto/streams/send.rs

@@ -1,8 +1,8 @@
-use http;
-use super::*;
 use codec::{RecvError, UserError};
 use codec::UserError::*;
 use frame::{self, Reason};
+use http;
+use super::*;
 
 use bytes::Buf;