py: use dict getter to retrieve binary

dict.get("foo") defaults to None if key "foo" is not found, which
is considered more Pythonic.

py/selenium/webdriver/firefox/webdriver.py

@@ -54,8 +54,7 @@ def __init__(self, firefox_profile=None, firefox_binary=None, timeout=30,
 
         # marionette
         if capabilities.get("marionette"):
-            if "binary" in capabilities:
-                self.binary = capabilities["binary"]
+            self.binary = capabilities.get("binary")
             self.service = Service(executable_path, firefox_binary=self.binary)
             self.service.start()