fix(DOS): cross pool dos vector removed by refunding eth only if weth…

[Alexangelj]

… token

Discussion: #89 (comment)

For pools with WETH, the _transferFrom will try to use the native ether for the payment if msg.value is >= the amount required.

If msg.value is not zero, but also not enough to make the payment, the else branch will be entered which will pull the full amount of WETH tokens from the sender.

If msg.value is greater than the amount (leaving excess), or the else branch was entered, the remaining ether balance of the contract is sent to the sender. Note this is only if the token is WETH,