NaN values?

[jedwards1211]

Since NaN !== NaN, a NaN value causes containSubset to fail, and the error message is confusing because it looks like it's erroneously complaining about missing fields...

Maybe we should use Object.is or equivalent?

      expect({foo: NaN, bar: 'baz'}).to.containSubset({foo: NaN})

      AssertionError: expected { foo: NaN, bar: 'baz' } to contain subset { foo: NaN }
      + expected - actual

       {
      -  "bar": "baz"
         "foo": NaN
       }