toString() for hash objects

[spangin]

Method toString() is used for hash oblects instead of hexString().
E.g.: https://github.com/HelixNetwork/testnet-1.0/blob/baf366ca574ce2079e83571d01047e7d73f3a9f1/src/main/java/net/helix/hlx/service/API.java#L988

To fix the problem it's possible to create toString() and return hex for all hashes as a result of this method or set hexString() instead of toString()

[oracle58]

should be fixed with #99