Encode URLs in utf-8 when escaping and unescaping

There is a problem while returning a path that has some special and possible Non-ASCII characters that may lead jekyll to break while doing the unescaping process. This is can be addressed by “forcing” ASCII to UTF-8.
Add tests to validate encoding of URLs

Added tests to validate the encoding of returned URL strings
after been escaped or unescaped.
Force encoding before escaping or unescaping

This will reassure not having any errors when escaping or
unescaping.
Change test to make it pass in Ruby 1.9.3
Revert changes and add encoding to the test file

* Add encoding to the test file as Ruby 1.9.3 doesn’t defaults to utf-8.
* Remove the forced encoding as encode seems too aggressive.
Update unescaped string test

Nothing was being tested without explicitly making the string
encoding ASCII.
Merge pull request #2420 from albertogg/fix-encoding-issue