Let ScrollSpy support uri encoded hash fragments

[yuku]

The updated line threw a syntax error when an encoded fragment is given.

href = encodeURIComponent('#テスト')
$(href)
// Error: Syntax error, unrecognized expression: %23%E3%83%86%E3%82%B9%E3%83%88

And browsers (at least Chrome 37.0.2062.124 and Firefox 32.0.3) automatically encodes a[href] values due to rfc3986:

<!-- server response -->
<a href="#テスト"></a>
<div id="テスト"></div>

<!-- browser recognizes as -->
<a href="#%23%E3%83%86%E3%82%B9%E3%83%88"></a>
<div id="テスト"></div>

This means that there is no way to use the plugin with non-ascii hash fragments. This PR fixes the problem.

Patched script works fine on my pages, however, I'm afraid of breaking something important because I'm not familiar with such encoding problems 😨
Thanks.

[cvrebert]

Please add a unit test for this feature.

[hnrch02]

This would probably be necessary for other plugins too.

[yuku]

@cvrebert Added a unit test. Please review fbf44e8

[hnrch02]

Please use four spaces for indentation in front of these pluses.

[yuku]

@hnrch02 Done. If I should merge these three commits into a single commit, please let me know.

[cvrebert]

@hnrch02 Should we group this in with #14789?

[hnrch02]

@cvrebert Yeah, I think so.

[hnrch02]

Closing this per my answer in #14789 (comment). We'll try to support all kinds of exotic (but still valid) IDs in v4. Thanks for the work though!