storage: fix signed url tests. fixes #238

- Removed the regression test
- Added a test that the expiration date we passed was honored by our code. Whether that's honored upstream or not is in the hands of another library's code :)
``` js
    it('should create a signed url', function(done) {
      bucket.getSignedUrl({
        action: 'read',
        expires: Math.round(Date.now() / 1000) + 5,
        resource: 'filename'
      }, function(err, signedUrl) {
        assert.ifError(err);
        assert.equal(typeof signedUrl, 'string');
        done();
      });
    });

    it('should use the provided expiration date', function(done) {
      var nowInSeconds = Date.now() / 1000;
      var expirationTimestamp = Math.round(nowInSeconds + 60);
      bucket.getSignedUrl({
        action: 'read',
        resource: 'filename',
        expires: expirationTimestamp
      }, function(err, signedUrl) {
        assert.ifError(err);
        var expires = url.parse(signedUrl, true).query.Expires;
        assert.equal(expires, expirationTimestamp);
        done();
      });
    });
```