fix: make intToHex produce valid Quantity

[josepot]

Fixes #70

[Gudahtt]

Thanks for the contribution! Completely agree with the "Quantity" data type not having the leading zero, that is wrong, but there were some cases where the leading zero was required. Not within an RPC payload, but when the hex string is used for certain operations that expect the input to have an even length (so that each pair of digits can be used to represent one byte).

I'm unsure if that concern applies in this case. I'll audit the uses of the intToHex funciton to find out.