Set (replace) and or get "Content-Type" header value

		:param value: value to set (if specified)
		:return: None if header doesn't exist, otherwise - str

def content_type(self, value=None):
		""" Set (replace) and or get "Content-Type" header value

		:param value: value to set (if specified)
		:return: None if header doesn't exist, otherwise - str
		"""
		content_type = self.normalize_name('Content-Type')
		if value is not None:
			self.replace_headers(content_type, value)
		if content_type in self.__headers.keys():
			return self.__headers[content_type][0]