Mutable reference for immutable AA

[Jack Applegame via Digitalmars-d-learn]

I made trivial pull request - 
https://github.com/D-Programming-Language/phobos/pull/3341

RebindableAA!(immutable int[string]) aa = ["a": 1, "b": 2]; // 
works
assert(aa["a"] == 1);  // cool
aa = ["a": 3, "b": 4]; // nice
auto bb = aa;          // yes
bb = ["a": 4, "b": 5]; // super

aa["a"] = 2;           // error, good
aa.remove("a");        // error, very good