XCDR2 set MU on keys only if explicitly marked MU

It turns out the problems with RTI's handling of the MU flag in mutable encoding are not
limited to XCDR1 ... See also a474791.

Signed-off-by: Erik Boasson <eb@ilities.com>

Author: eboasson

cyclonedds/idl/_machinery.py

@@ -1118,7 +1118,10 @@ def serialize(self, buffer, value, serialize_kind=SerializeKind.DataSample, key_
 
             buffer.align(4)
             if self.use_version_2:
-                buffer.write('I', 4, mutablemember.header | ((1 if m_key_enabled != KeyEnabled.Never else 0) << 31))
+                # It appears RTI also can't handle "must understand" on key fields in
+                # XCDR2, even though XTypes 1.3 requires it.  This should add:
+                #   | ((1 if m_key_enabled != KeyEnabled.Never else 0) << 31)
+                buffer.write('I', 4, mutablemember.header)
             else:
                 # TODO: use compact variant when member id and the max serialized size of the type are small enough
                 buffer.write('H', 2, XCDR1.PL_SHORT_PID_EXTENDED | XCDR1.PL_SHORT_FLAG_MU)