Fix interaction with WebKitDriver

[jonhoo]

Originally reported in #36.
Running fantoccini against WebKitDriver currently fails with

SessionNotCreated(WebDriverError {
  error: SessionNotCreated,
  message: "Failed to match capabilities",
  stack: "",
  delete_session: false
})

This seems to stem from this line in the WebKitDriver source (GH mirror link), but I cannot for the life of me see why matchedCapabilitiesList would be empty. Setting browserName or other parameters in the alwaysMatch capabilities list does not seem to make a difference. I think this might be a bug in WebKitDriver, but some more investigation is probably warranted.