@debug does not print map entries when the key is null

[chriseppstein]

$m: (foo: 1px, null: 2px);
@debug $m;
@debug map-get($m, null);

Prints out:

stdin:1: DEBUG: (foo: 1px)
stdin:2: DEBUG: 2px

[chriseppstein]

More fun stuff about @debug:

$m: (foo: 1px, null: 2px, false: 3px, true: 4px);
@debug $m;
@debug map-get($m, null);
@debug map-get($m, false);
@debug null;

prints out:

stdin:1: DEBUG: (foo: 1px, true: 4px)
stdin:2: DEBUG: 2px
stdin:3: DEBUG:
stdin:4: DEBUG:

Should print out:

Line 2 DEBUG: (foo: 1px, null: 2px, false: 3px, true: 4px)
Line 3 DEBUG: 2px
Line 4 DEBUG: 3px
Line 5 DEBUG: null

[xzyfer]

Thanks @chriseppstein this should be a straight forward fix.

[xzyfer]

As an aside I've noticed the following now produces an error

$m: (foo: 1px, null: 2px, false: 3px, true: 4px);

Error: Duplicate key "false" in map (foo: 1px, true: 4px).
        on line 1 of test.scss
>> $m: (foo: 1px, null: 2px, false: 3px, true: 4px);

[xzyfer]

Spec added sass/sass-spec#437

[xzyfer]

This is fixed, and will be in 3.3.

An additional aspect to this issue has been broken out into #1336