Fix props field for witness_set_properties_opeation

Please review: @mkudela @mzebrak

Props in witness_set_properties_operation are of this form: [['account_creation_fee', '606d0000000000000354455354530000'], ['account_subsidy_budget', '1d030000'], ['account_subsidy_decay', 'b94c0500'], ['key', '02410be8f7ca66c250f420a07382ba23af572c98f5fc825ce73d24b7ace17e0e6d'], ['maximum_block_size', '00000200'], ['new_signing_key', '02410be8f7ca66c250f420a07382ba23af572c98f5fc825ce73d24b7ace17e0e6d'], ['sbd_exchange_rate', 'a0860100000000000354424400000000a0860100000000000354455354530000'], ['sbd_interest_rate', 'e803'], ['url', '0f687474703a2f2f75726c2e68746d6c']]. This MR provides validation of this field.