Add missing requirements to ERC777 (#2212)

* Restore _approve check for zero sender

* Add non-zero operator check to _send

contracts/mocks/ERC777Mock.sol

@@ -23,4 +23,8 @@ contract ERC777Mock is Context, ERC777 {
     ) public {
         _mint(operator, to, amount, userData, operatorData);
     }
+
+    function approveInternal(address holder, address spender, uint256 value) public {
+        _approve(holder, spender, value);
+    }
 }