RENAME should raise an error if the old key is missing

[xavdid]

Per the docs, rename('a', 'b') should throw an error if 'a' doesn't exist. Currently, it's returning False:

mockredis/mockredis/client.py

Lines 282 to 288 in ba41d90

	def _rename(self, old_key, new_key, nx=False):
	old_key = self._encode(old_key)
	new_key = self._encode(new_key)
	if old_key in self.redis and (not nx or new_key not in self.redis):
	self.redis[new_key] = self.redis.pop(old_key)
	return True
	return False

Happy to try for a PR, though I'm not super familiar with python modules and it's probably an easy fix for someone more experienced