[ecashaddrjs] Return type as lowercase for use with chronik

Summary: Originally returned the values as uppercase to be consistent with return of cashaddr.decode without call for true. However, this function is only used to make chronik calls. The all uppercase type always needs to be .toLowerCase()'d in apps where this function is used. So, should fix it here.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14163