Return header value by its name

		:param header_name: header name
		:return: tuple of str

def get_headers(self, header_name):
		""" Return header value by its name

		:param header_name: header name
		:return: tuple of str
		"""
		header_name = self.normalize_name(header_name)
		if header_name in self.__headers.keys():
			return tuple(self.__headers[header_name])