Map and Set don't work with concatenated strings

[cleiner]

I have been trying to run some third-party code on Rhino 1.7.11 and noticed that Map and Set don't always behave as expected. Here are two test cases to reproduce the issue:

  var myMap = new Map([['key1', 'value1'], ['key2', 17]]);
  for(let i = 1; i <= myMap.size; i++) {
    let key = 'key' + i;
    print(myMap.get(key));
  }

Expected output: value1, 17 | Actual output: undefined, undefined

var mySet = new Set(['word1', 'word2']);
for(let i = 0; i <= mySet.size; i++) {
  let value = 'word' + i;
  print(mySet.has(value));
}

Expected output: false, true, true | Actual output: false, false, false

Due to the string concatenation, Rhino passes a ConsString instance to the NativeMap/NativeSet methods which is of course not equal to the String that was added to the collection initially.

Unlike #247 and #496 there's no obvious or intentional Java-interop here, so I hope this warrants a separate issue.

[rbri]

Will try to fix that