Fix escaping of interpolated attribute values

[sinkuu]

Attribute values with interpolated strings are not totally escaped.

a(href="/?t=1&u=1&v=1") foo
- auto params = "t=1&u=1";
a(href="/?#{params}&v=1") foo

<a href="/?t=1&amp;u=1&amp;v=1">foo</a>
<a href="/?t=1&amp;u=1&v=1">foo</a>

[s-ludwig]

Thanks! Interestingly, the !{...} style interpolations also actually don't really make sense in this context. I'll add an error message for the case of !escape && attribute.