Allow whitespace before method call in assert_select_jquery.

In Javascript, whitespace is permitted between function calls, like so:

```js
$('#new_subscription')
  .append("<p class=\"message success\"><i class=\"fa fa-check\"></i> You&rsquo;ve been added to the mailing list. Thanks!</p>")
  .find('input[type=email]')
  .val('');
```

The following assertion using `assert_select_jquery` fails on the above example with the latest version of the `jquery-rails` gem:

 ```ruby
assert_select_jquery :append, '#new_subscription'
```

The above assertion fails because `assert_select_jquery` assumes no whitespace between the jQuery call and the `append` method call.  This pull request modifies the regular expression generated in `assert_select_jquery` to permit legal whitespace.