Loss of unused Ether in meta-transaction

[aaaooie]

In the MinimalForwarder.execute(ForwardRequest req, bytes signature) payable function, the msg.value may contain more Ether than specified in req.value. In this case (since only req.value is transferred further), the unused amount of Ether will remain on the contract address.

[frangio]

Thank you @aaaooie. As mentioned in the comments:

openzeppelin-contracts/contracts/metatx/MinimalForwarder.sol

Lines 12 to 15 in 4b8b92c

	* MinimalForwarder is mainly meant for testing, as it is missing features to be a good production-ready forwarder. This
	* contract does not intend to have all the properties that are needed for a sound forwarding system. A fully
	* functioning forwarding system with good properties requires more complexity. We suggest you look at other projects
	* such as the GSN which do have the goal of building a system like that.

MinimalForwarder is inded too minimal. So much so I think we might be better off removing it.