fix(conn): always flush io from poll_complete

Closes #1108

src/http/conn.rs

@@ -827,6 +827,7 @@ mod tests {
             assert!(conn.state.writing.is_queued());
             assert!(conn.poll_complete().unwrap().is_ready());
             assert!(!conn.state.writing.is_queued());
+            assert!(conn.io.io_mut().flushed());
 
             Ok(())
         }).wait();

src/http/io.rs

@@ -134,15 +134,16 @@ impl<T: Write> Write for Buffered<T> {
 
     fn flush(&mut self) -> io::Result<()> {
         if self.write_buf.remaining() == 0 {
-            Ok(())
+            self.io.flush()
         } else {
             loop {
                 let n = try!(self.write_buf.write_into(&mut self.io));
                 debug!("flushed {} bytes", n);
                 if self.write_buf.remaining() == 0 {
-                    return Ok(())
+                    break;
                 }
             }
+            self.io.flush()
         }
     }
 }