Fix interaction with WebKitDriver

Originally reported in https://github.com/jonhoo/fantoccini/issues/36.
Running fantoccini against [WebKitDriver](https://trac.webkit.org/browser/webkit/trunk/Source/WebDriver) currently fails with
```
SessionNotCreated(WebDriverError {
error: SessionNotCreated,
message: "Failed to match capabilities",
stack: "",
delete_session: false
})
```

This seems to stem from [this line](https://trac.webkit.org/browser/webkit/trunk/Source/WebDriver/WebDriverService.cpp?rev=234837#L628) in the WebKitDriver source ([GH mirror link](https://github.com/WebKit/webkit/blob/48b3b6748f3a65d95ffd3a5633e1093299cba14b/Source/WebDriver/WebDriverService.cpp#L628)), 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.

Reproduce the session-creation failure with WebKitDriver, then inspect WebKitDriver's WebDriverService.cpp around line 628 and fantoccini's capability handling. Done means determining whether the incompatibility is in fantoccini or WebKitDriver and documenting the finding or identifying the required interaction change.

Written by the indexing model from the issue text.