py: test for existence of marionette key in capabilities

The existing key checked for strict boolean equivalence, but using
surprising Python syntax (foo is True, rather than foo == True).
This changes it to be a loose truthyness check as suggested by
@Dude-x.

py/selenium/webdriver/firefox/webdriver.py

@@ -53,7 +53,7 @@ def __init__(self, firefox_profile=None, firefox_binary=None, timeout=30,
             capabilities = DesiredCapabilities.FIREFOX
 
         # marionette
-        if "marionette" in capabilities and capabilities["marionette"] is True:
+        if capabilities.get("marionette"):
             if "binary" in capabilities:
                 self.binary = capabilities["binary"]
             self.service = Service(executable_path, firefox_binary=self.binary)