Test coverage of existing methods to check the input parsing

[khorolets]

During the last release, we messed up the input parsing and accidentally dropped the support for some legacy APIs.

This is unacceptable since the ReadRPC must be fully compatible with the native NEAR RPC. I think we should add some tests.

Initially, we should test that we have kept existing input parsing intact to prevent this from happening again. Eventually, we should cover everything possible.