PHP json serializer skips maps with int/bool keys

When the PHP generator runs with the "json" option, it skips fields that are maps with non-string keys. An instance of this struct will produce an empty object when JSON-serialized:

struct Foo {
  1: map<i32,string>;
}

This is unnecessarily strict, since PHP's json_encode() function will convert the keys to strings for you.

Pull request coming.