[py] add headless convenience method for Firefox Options

py/selenium/webdriver/firefox/options.py

@@ -110,6 +110,25 @@ def add_argument(self, argument):
            raise ValueError()
        self._arguments.append(argument)

    @property
    def headless(self):
        """
        Returns whether or not the headless argument is set
        """
        return '-headless' in self._arguments

    def set_headless(self, headless=True):
        """
        Sets the headless argument

        Args:
          headless: boolean value indicating to set the headless option
        """
        if headless:
            self._arguments.append('-headless')
        elif '-headless' in self._arguments:
            self._arguments.remove('-headless')

    def to_capabilities(self):
        """Marshals the Firefox options to a `moz:firefoxOptions`
        object.