Fix trimmed zeros in transaction encoding

[alisinabh]

We don't need to trim zeros if we:

1. Make sure the transaction data will always be hex encoded starting with '0x' AND
2. Pass them as integers to ExRLP.encode/1

Using the existing trim_leading/1 on all transaction values is dangerous as it can alter the transaction. For example a transaction data starting with one or more zero(s) or transaction to address which start with one or more zero(s) is perfectly fine. Since ExRLP already handles integers, we need to pass these transaction parameters as integers so we get the correct RLP encoding.

Closes #91

[wchenNL]

🚢 LGTM