Make SendStream::poll_capacity never return Ok(Some(0)) (#596)

Fixes #270

tests/h2-support/src/util.rs

@@ -32,6 +32,7 @@ pub async fn yield_once() {
     .await;
 }
 
+/// Should only be called after a non-0 capacity was requested for the stream.
 pub fn wait_for_capacity(stream: h2::SendStream<Bytes>, target: usize) -> WaitForCapacity {
     WaitForCapacity {
         stream: Some(stream),
@@ -59,6 +60,11 @@ impl Future for WaitForCapacity {
 
             let act = self.stream().capacity();
 
+            // If a non-0 capacity was requested for the stream before calling
+            // wait_for_capacity, then poll_capacity should return Pending
+            // until there is a non-0 capacity.
+            assert_ne!(act, 0);
+
             if act >= self.target {
                 return Poll::Ready(self.stream.take().unwrap().into());
             }