extended_asset and extended_symbol are not returned as a formatted string when read from tables

[prcolaco]

Version of EOSJS
any

Describe the bug
When using these extended types (extended_asset and extended_symbol) one must use a string like '100.0000 EOS@eosio.token', or '4,EOS@eosio.token' when passing as arguments to actions, but the inverse conversion is not made when reading them from tables, as opposed to regular asset and symbol types which are converted back to string format as expected.

To Reproduce
Steps to reproduce the behavior:

1. Push an action that stores an eosio::extended_xxx in a table
2. Read from said table the field and receive a JSON object instead of a string (as occurs in asset or symbol)

Expected behavior
It was expected to have similar behaviour as their regular counter parties asset and symbol. If you send a string, it is expected to receive a string in the same format back when reading.

Desktop (please complete the following information):
any

Smartphone (please complete the following information):
any

Additional context
It seems that the conversion implementation on the reading part was missed when adding support to these extended types.