Disallow ERC20._transfer from the zero address. (#1752)

* Add requirement of non-zero from to ERC20 transfer.

* Add test for transferFrom zero address to behavior.

* Create ERC20.transfer behavior.

* Add tests for _transfer.

* Add changelog entry.

* Fix linter error.

* Delete repeated test.

* Fix hardcoded error prefix.

* Update CHANGELOG.md

Co-Authored-By: Francisco Giordano <frangio.1@gmail.com>

* Address review comments.

contracts/token/ERC20/ERC20.sol

@@ -125,6 +125,7 @@ contract ERC20 is IERC20 {
      * @param value The amount to be transferred.
      */
     function _transfer(address from, address to, uint256 value) internal {
+        require(from != address(0), "ERC20: transfer from the zero address");
         require(to != address(0), "ERC20: transfer to the zero address");
 
         _balances[from] = _balances[from].sub(value);