Fix :wrapper_html :class and :button_html :class options

This appears to fix the options functionality retaining both :wrapper_html and :button_html options without actually wrapping the action in another tag.
Only tested with class and ensured that id also remains but not sure what it does with other options.

Default class is "btn"
:wrapper_html => { :class => "wrapper-option" } is prefixed with the default and results in class="btn wrapper-option"
:button_html => { :class => "button-option" } will specify the whole class value and results in class="button-option"
Merge pull request #78 from nickl-/master

Fix :wrapper_html :class and :button_html :class options