Remove assert around self.pending_capacity.is_empty() (#225)

This assert does not hold as many streams can be pushed into
pending_capacity during a call to send_data(). See issue #224
for more discussion and sign-off.

Closes #224

src/proto/streams/prioritize.rs

@@ -353,9 +353,6 @@ impl Prioritize {
 
         // First check if capacity is immediately available
         if conn_available > 0 {
-            // There should be no streams pending capacity
-            debug_assert!(self.pending_capacity.is_empty());
-
             // The amount of capacity to assign to the stream
             // TODO: Should prioritization factor into this?
             let assign = cmp::min(conn_available, additional);