Don't freeze nil.

In Ruby, the nil singleton object is not frozen. Freezing it breaks
some other gems.

~$ irb
2.1.1 :001 > nil.frozen?
 => false

Conveniently, the `unless` syntax leaves `@value` as good ol' nil,
so there's no need for a ternary.
Improve the warning message for index(...) == false.
Fix the use of unknown directives within @at-root.

Closes #1174
Fix a typo in the adjust-hue() documentation.

Closes #1180
Fix a bug in the index() documentation.
Merge branch 'stable' into do-not-freeze-nil
Add a changelog entry for freezing nil.
Merge branch 'do-not-freeze-nil' into stable
Fix a Windows path canonicalization bug.
Merge remote-tracking branch 'origin/stable' into stable

Conflicts:
	test/sass/scss/scss_test.rb