parse `class` attr's text value before concatenate

    %span.badge{:class => "#{$foo->bar}"}
    ## was
    <span class='#{Foo=HASH(0x1b0c258)->foo} badge'></span>
    ## to be
    <span class='badge baz'></span>
Merge pull request #25 from aanoaa/class-attr-interpolation

parse `class` attr's text value before concatenate