Fix conversion of null.

Closes #709
Merge branch 'stable'
Make null.to_sass take an options hash.
Fix an invalid mixin-loop report.

Closes #714
Merge branch 'stable'
Fix running engine_test on its own.
Merge remote-tracking branch 'origin/stable' into stable
Merge branch 'stable'
Bump VERSION to 3.2.8.
Merge branch 'stable'

Conflicts:
	VERSION
	doc-src/SASS_CHANGELOG.md
Merge remote-tracking branch 'origin/master'
Fix a mutability bug.

Closes #722.
Merge branch 'stable'

Conflicts:
	doc-src/SASS_CHANGELOG.md
Explicitly validate that script test return values are SassScript objects.
Merge branch 'stable'

Conflicts:
	test/sass/functions_test.rb
Fix list-separator for varargs.
Don't load guard/listen 1.0.

Closes #719
Update the vendored listen to 0.7.3.
Merge branch 'stable'

Conflicts:
	lib/sass/plugin/compiler.rb
Stop bundling listen with Sass.

This was already documented in the changelog, but somehow managed to escape
having the change itself committed.