Gift card fund hexlify issue with xDAI transfer

[Cafe137]

Our Rpc.sendNativeTransaction implementation needs a string for the value of the transaction.

This was this.dai.toBigNumber.minus(totalCost.toString()).toString() before. I tried out the gift card fund feature with the fake-bee and even though it is a mock environment, I ran into an issue which seemed to be client side only in Bee Dashboard.

This string is provided wrongly, and the correct expression may be '0x' + this.dai.toBigNumber.minus(totalCost.toString()).toString(16), that is hexademical and leading 0x (ethers hexlify).

This should be investigated with a proper mainnet gift card fund manual test. BTW I attempted to transfer 13.9529 xDAI