Support named args in method calls

[whelks-chance]

res = self.contract.get_transaction_statuses(txn_hash, opts={'fields': 'transaction_hash,state'})
at the moment this throws
TypeError: _auth_required_fn_wrapper() got an unexpected keyword argument 'opts'

from the auth decorator.

res = self.contract.get_transaction_statuses(txn_hash, {'fields': 'transaction_hash,state'})

works fine.