code: def with_header(self, key, value):
          
          self.header[canonicalize_header(key)] = value
          return self
docs: Sets a header on the request and returns the request itself.
          The header key will be canonicalized before use. (see also: canonicalize_header)
  
          Keyword arguments:
          key -- the header's name
          value -- the string value for the header