Update Cache.put() test to accept string as request

Per spec, the method signature is:

Promise<Response> put((Request or USVString) request, Response response);

So passing a string for request is perfectly valid. And thanks to the
wonders of Web IDL + JavaScript, anything else passed in other than a
Request will be coerced to a string, too.

R=asanka@chromium.org
BUG=374802

Review URL: https://codereview.chromium.org/677533002

git-svn-id: svn://svn.chromium.org/blink/trunk@184371 bbb929c8-8fbe-4397-9dbb-9b2b20218538