Escape URL by ERB::Util.u. 

As I explained in rails-taiwan/railsbridge-docs#6, it seems that [`ERB::Util.url_encode`](http://ruby-doc.org/stdlib-2.1.2/libdoc/erb/rdoc/ERB/Util.html#method-c-url_encode) works great for non-ASCII and the `:` characters. I think we should migrate to it, although it requires ERB library.

Hopefully fixes the issues of broken `:` escaping after #353 was merged, which was reverted at 01ca022f14fd03095bfe49ba13ede55e82409f38.