reduce available window capacity when settings lowers window size

src/proto/streams/flow_control.rs

@@ -126,8 +126,10 @@ impl FlowControl {
     /// This is called after receiving a SETTINGS frame with a lower
     /// INITIAL_WINDOW_SIZE value.
     pub fn dec_window(&mut self, sz: WindowSize) {
+        trace!("dec_window; sz={}; window={}, available={}", sz, self.window_size, self.available);
         // This should not be able to overflow `window_size` from the bottom.
         self.window_size -= sz as i32;
+        self.available = self.available.saturating_sub(sz);
     }
 
     /// Decrements the window reflecting data has actually been sent. The caller