Improve test coverage of createSet

[mroderick]

This PR adds explicit tests of createSet to get it's coverage to 100%.

Purpose (TL;DR) - mandatory

Trying to get test coverage to 100%, so we can lock it there.

Solution - optional

• Upgrade @sinonjs/referee to latest to get the isSet assertion
• Add tests

How to verify - mandatory

1. Check out this branch
2. npm install
3. Put a .only in the first describe in create-set.test.js
4. npm run test-coverage
5. Observe that the test coverage of createSet is 100%, when only its tests are run

-----------------------|----------|----------|----------|----------|-------------------|
File                   |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
-----------------------|----------|----------|----------|----------|-------------------|
All files              |    31.78 |     4.78 |     8.41 |    31.78 |                   |
 create-set.js         |      100 |      100 |      100 |      100 |                   |
 deep-equal.js         |       26 |        0 |    16.67 |       26 |... 32,233,240,252 |
 get-class.js          |    66.67 |      100 |        0 |    66.67 |                 9 |
 identical.js          |       50 |        0 |        0 |       50 |          18,19,22 |
 is-arguments.js       |    15.38 |        0 |        0 |    15.38 |... 26,27,28,30,32 |
 is-date.js            |       50 |      100 |        0 |       50 |                 4 |
 is-element.js         |    22.22 |    28.57 |        0 |    22.22 |... 18,19,20,22,24 |
 is-map.js             |       50 |        0 |        0 |       50 |                 4 |
 is-nan.js             |    33.33 |        0 |        0 |    33.33 |               7,8 |
 is-neg-zero.js        |       50 |        0 |        0 |       50 |                10 |
 is-object.js          |       50 |        0 |        0 |       50 |                11 |
 is-set.js             |       50 |        0 |        0 |       50 |                 4 |
 is-subset.js          |       20 |        0 |        0 |       20 |... ,9,10,11,14,17 |
 iterable-to-string.js |    23.53 |        0 |        0 |    23.53 |... 33,34,37,38,41 |
 match.js              |    19.35 |        0 |    33.33 |    19.35 |... 19,123,126,129 |
 matcher.js            |    32.73 |     6.72 |     6.49 |    32.73 |... 47,454,455,458 |
 samsam.js             |      100 |      100 |      100 |      100 |                   |
-----------------------|----------|----------|----------|----------|-------------------|

Checklist for author

• npm run lint passes
• References to standard library functions are cached.

[coveralls]

Pull Request Test Coverage Report for Build 310

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 98.389%

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Totals
Change from base Build 308:	0.4%
Covered Lines:	465
Relevant Lines:	472

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💛 - Coveralls