Method Approve is vulnerable

[pauliax]

Approve method is vulnerable to the issue described here: https://docs.google.com/document/d/1YLPtQxZu1UAvO9cZ1O2RPXBbT0mooh4DYKjA_jp-RLM/edit#heading=h.m9fhqynw2xvt
please consider adding this check at the beginning of the method:
// To change the approve amount you first have to reduce the addresses// allowance to zero by callingapprove(_spender, 0)` if it is not
// already 0 to mitigate the race condition described here:
// ethereum/EIPs#20 (comment) …

require((_value == 0) || (allowed[msg.sender][_spender] == 0));

[shoaib-dr]

This was recently removed as a part of the patch to the OpenZeppelin framework under the pretext that this violates the ERC20 standards.

More details on OpenZeppelin/openzeppelin-contracts@83918ca