fix: Attribute closes the quotations prematurely

[dcyriller]

A fix for #371

I couldn't write to #371, so I'm opening this new PR to fix the failing test. Thanks a lot @kamal for the report and the failing test. It was very helpful!

The Bug

As @kamal wrote in the description:

When an attribute contains newlines, a mutation to a MustacheStatement inside of it will cause printing the AST to prematurely close the quotations, causing invalid markup.

Example,

<div
  class="
    block
    {{if this.foo "bar"}}
  "
>
  hello
</div>

when parsed, mutated and printed will produce

<div
  class=""
    block
    {{if this.foo "bar"}}
  "
>
  hello
</div>