remove SafeBuffer from link_to helper options, fixes #1529, add tests

The other way to fix the problem is to make `TagHelpers#tag_attributes` https://github.com/padrino/padrino-framework/blob/master/padrino-helpers/lib/padrino-helpers/tag_helpers.rb#L242 and `TagHelpers#nested_values` https://github.com/padrino/padrino-framework/blob/master/padrino-helpers/lib/padrino-helpers/tag_helpers.rb#L267 understand SafeBuffer. I tried it and it works fine too.