fix datetime json serialization

[DusanFedorcak]

I stumbled upon another serialization error. This

from terra_sdk.client.lcd import LCDClient
from terra_sdk.util.hash import hash_amino

terra = LCDClient(chain_id="columbus-5", url="https://lcd.terra.dev")
bi = terra.tendermint.block_info(7309740)
info = terra.tx.tx_info(hash_amino(bi["block"]["data"]["txs"][143]))
info.tx.body.messages[0].to_json()

raises

TypeError: Object of type datetime is not JSON serializable
at terra_sdk/util/json.py:58

The fix is below but I'm not sure about the general solution (turning a datetime into a str as to_data seems to be used on many places)

Also, I believe that the terra_sdk.util.converter.to_isoformat() has a small bug
I was not able to parse it back with dateutils unlet the "dot" is removed:

dateutil.parser.isoparse(json.loads(info.tx.body.messages[0].to_json())["allowance"]["expiration"])