set_multi, get_multi, delete_multi raise KeyError when used with tuple keys

[sergiimk]

We are relying on feature that allows specifying keys as tuples (hashvalue, key).

After upgrading from version 1.43 to 1.59 the *_multi methods started railing KeyError when used with tuple keys.

I traced this problem down to this line which seemingly unnecessarily mutates the orig_key, that makes the original key not present in resulting prefixed_to_orig_key.