Fix bug in prioritization (#63)

The stream buffered data counter was never decremented.

tests/support/src/assert.rs

@@ -6,6 +6,26 @@ macro_rules! assert_closed {
     }}
 }
 
+#[macro_export]
+macro_rules! assert_headers {
+    ($frame:expr) => {{
+        match $frame {
+            ::h2::frame::Frame::Headers(v) => v,
+            f => panic!("expected HEADERS; actual={:?}", f),
+        }
+    }}
+}
+
+#[macro_export]
+macro_rules! assert_data {
+    ($frame:expr) => {{
+        match $frame {
+            ::h2::frame::Frame::Data(v) => v,
+            f => panic!("expected DATA; actual={:?}", f),
+        }
+    }}
+}
+
 #[macro_export]
 macro_rules! assert_ping {
     ($frame:expr) => {{