Fix two issues with no-misspelled-properties rule

[bgriffith]

What do the changes you have made achieve?

Fix two issues ( #352, #679 ) related to the no-misspelled-properties rule

Are there any new warning messages?

No

Have you written tests?

Yes

Have you included relevant documentation

N/A

Which issues does this resolve?

Closes #352
Closes #679

<DCO 1.1 Signed-off-by: Ben Griffith gt11687@gmail.com>

[coveralls]

Coverage increased (+0.02%) to 97.112% when pulling 81bb14b on bgriffith:feature/no-miss-props into 098de67 on sasstools:develop.

[bgriffith]

Need to update since older versions of node don't handle Object.assign.

[DanPurdy]

Use Array.concat instead

[coveralls]

Coverage increased (+0.02%) to 97.112% when pulling 50a859d on bgriffith:feature/no-miss-props into 6b7211a on sasstools:develop.

[DanPurdy]

👍