Compressed output adds extra ; for unknown at-directives

[xzyfer]

Discovered by failing sass spec tests with Libsass. The extra ; is only present in compressed mode.

foo {
  @foo;
  a: b;
}

Produces

foo{@foo;;a:b}

Expected

foo{@foo;a:b}

[xzyfer]

An extension of this issue

foo {
  @foo;
  a: b;
  @bar;
}

Produces

foo{@foo;;a:b;@bar;}

Expected

foo{@foo;a:b;@bar}

The CSS 2.1 spec states

An at-rule consists of everything up to and including the next semicolon (;) or the next block, whichever comes first.

• Source

Which suggests the trailing ; in at-rules is optional.