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/store.rs

@@ -215,10 +215,6 @@ where
         }
     }
 
-    pub fn is_empty(&self) -> bool {
-        self.indices.is_none()
-    }
-
     pub fn take(&mut self) -> Self {
         Queue {
             indices: self.indices.take(),