Test all frames are writable

User code working with NumPy or Pandas objects often expects the objects
to be mutable. However if read-only frames (like `bytes`) objects are
used, this is not true. So add a test to check for this so that we can
make sure this is true and we can catch and fix cases where that may not
be true.

distributed/protocol/tests/test_protocol_utils.py

@@ -25,6 +25,7 @@ def test_merge_frames(lengths, frames):
         data = data[i:]
 
     assert all(isinstance(f, memoryview) for f in result)
+    assert all(not f.readonly for f in result)
     assert list(map(ensure_bytes, result)) == expected