Describe the bug
#309 changed some RPC APIs to accept Uint160 parameters as either base58-based address or 20-byte hex string. While in general this is a good thing (making APIs more useful for clients), one API stands out a little and that is validateaddress. In my understanding of it, it was specifically designed to check base58-encoded address for correctness (I'm not sure it's very useful, but that's what it does), so for it any non-base58 form of Uint160 is an invalid input and it shouldn't accept it.

Expected behavior
Revert the change for validateaddress and return its historic behavior.