approve should not enforce setting to 0 value

[rstormsf]

LOW IMPORTANCE MiniMeToken.approve(...) has a check requiring non-zero approval limits to be set to 0 before being set to a different non-zero value. The recently finalised ERC20 Token Standard suggest that the contract should not enforce this requirement

From @bokky audit https://github.com/bokkypoobah/AigangCrowdsaleContractAudit/tree/master/audit#testing