Update capabilities properly instead of assuming dict structure. Fixes …

…SeleniumHQ#3927

We were previously assuming, incorrectly, the shape of the dict. We then
use the key that we wanted originally... *sigh* I should have been more
careful.

py/selenium/webdriver/remote/webdriver.py

@@ -181,7 +181,7 @@ def start_session(self, capabilities, browser_profile=None):
             if "moz:firefoxOptions" in capabilities:
                 capabilities["moz:firefoxOptions"]["profile"] = browser_profile.encoded
             else:
-                capabilities['desiredCapabilities'].update({'firefox_profile': browser_profile.encoded})
+                capabilities.update({'firefox_profile': browser_profile.encoded})
         w3c_caps["alwaysMatch"].update(capabilities)
         parameters = {"capabilities": w3c_caps,
                       "desiredCapabilities": capabilities}