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.
Ensure atomic writes are fully flushed to disk.
Bump VERSION to 3.2.17.
Remove whitespace
Fix some typos
Merge branch 'stable_3_2' without the changes.

Recent changes to stable_3_2 are not needed on stable, this merge
ensures we won't try to merge them in at a later point.
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
Atomic write permissions should default to 0644.

Temporary files in ruby default to very conservative permissions. Since
atomic writes start their life as a temporary file, this caused the
resulting file to have the permissions of a temporarily file instead
of the permissions you'd expect by File.open.

This patch makes sure that atomic writes are the same as normal file
permissions which is basically 0666 and then masked by the umask.
Merge pull request #1182 from jwworth/task/fix_typos_and_whitespace

Task/fix typos and whitespace
Merge branch 'stable_3_2' into stable

Conflicts:
	lib/sass/util.rb
Merge remote-tracking branch 'nex3/stable' into stable
Fix a Windows path canonicalization bug.
Merge remote-tracking branch 'origin/stable' into stable

Conflicts:
	test/sass/scss/scss_test.rb
Bump VERSION to 3.3.4.