Return handler which scheme name matches the specified one

		:param scheme_name: scheme name to search for
		:return: WSchemeHandler class or None (if matching handler was not found)

def handler(self, scheme_name=None):
		""" Return handler which scheme name matches the specified one

		:param scheme_name: scheme name to search for
		:return: WSchemeHandler class or None (if matching handler was not found)
		"""
		if scheme_name is None:
			return self.__default_handler_cls
		for handler in self.__handlers_cls:
			if handler.scheme_specification().scheme_name() == scheme_name:
				return handler