Maps allow integers as keys

[stephenkeatley-eagleeye]

Added support so maps allow both strings and ints as keys.

Keys in PHP arrays will automatically cast to integers if all characters are integer digits. This change allows both strings and ints in the validation and then casts the key to a string when doing the actual encoding.

e.g.

{
  "mapContainer": {
    123: "value1",    // invalid before because it's an int, ok now
    "456": "value2",  // invalid before because array key gets converted to int, ok now
    "myKey": "value4" // ok before and now, stays as a string because not all int characters
  }
}