Fix deserialization failure when fetching contract source_code from blockscout

[ckoopmann]

Blockscouts response upon requesting a contracts source code diverges from that of etherscan and is thereby incompatible with the current serde deserialization schema:

1. Use "true" / "false" instead of "1"/"0" for various binary fields (e.g. "OptimizationUsed")
2. Different naming of certain fields (e.g. "IsProxy" vs. "Proxy")
3. Omitting fields entirely when data would be empty instead of returning an empty string (e.g. "ConstructorArguments").

For reference compare the blockscout and etherscan example response for the ETH deposit contract.

This PR fixes that by adjusting the serde deserialization options accordingly.

This partially addresses #21

[Evalir]

this is only for fetching contract sources, so it should be alright. Thanks!