Updates the dictionary with a single header line.

        >>> h = HTTPHeaders()
        >>> h.parse_line("Content-Type: text/html")
        >>> h.get('content-type')
        'text/html'

def parse_line(self, line: str) -> None:
        """Updates the dictionary with a single header line.

        >>> h = HTTPHeaders()
        >>> h.parse_line("Content-Type: text/html")
        >>> h.get('content-type')
        'text/html'
        """
        if line[0].isspace():
            # continuation of a multi-line header
            if self._last_key is None:
                raise HTTPInputError("first header line cannot start with whitespace")
            new_part = " " + line.lstrip()
            self._as_list[self._last_key][-1] += new_part
            self._dict[self._last_key] += new_part
        else:
            try:
                name, value = line.split(":", 1)
            except ValueError:
                raise HTTPInputError("no colon in header line")
            self.add(name, value.strip())