PYTHON-1014 - Omit $readPreference from find command

Author: behackett

pymongo/message.py

@@ -175,6 +175,8 @@ def _gen_find_command(coll, spec, projection, skip, limit, batch_size,
  for key, val in spec.items()])
  if '$explain' in cmd:
  cmd.pop('$explain')
+ if '$readPreference' in cmd:
+ cmd.pop('$readPreference')
  else:
  cmd['filter'] = spec