[BUG] Receipt Merchant Data Not Saving

Describe the bug
Merchant data not added when details entered

To Reproduce

from monzo.handlers.filesystem import FileSystem
from monzo.endpoints.receipt import Receipt, ReceiptItem
from monzo.authentication import Authentication

EXTERNAL_ID = 'ABC123'
TRANSACTION_ID = 'REPLACE WITH REAL ID'

handler = FileSystem(file='monzo.json')
creds = handler.fetch()
redirect_url = ''
auth = Authentication(
    access_token=creds['access_token'],
    access_token_expiry=creds['expiry'],
    client_id=creds['client_id'],
    client_secret=creds['client_secret'],
    redirect_url=redirect_url,
    refresh_token=creds['refresh_token'],
)
auth.register_callback_handler(handler=handler)

item = ReceiptItem(
    description = 'test',
    amount= 1234,
    currency = 'GBP',
    tax=123
)

receipt = Receipt(
    auth = auth,
    transaction_id = TRANSACTION_ID,
    external_id = EXTERNAL_ID,
    transaction_total = 1234,
    transaction_currency = 'GBP',
    items=[item]
)

receipt.add_tax(description='some dummy description', amount=12334, currency='USD')

receipt.add_merchant(name='test123', online=True)

Receipt.create(auth=auth, receipt=receipt)
rec = Receipt.fetch(auth=auth, external_id=EXTERNAL_ID)
print(rec)

Expected behavior
Merchant data to show in the Monzo app and to be returned with a fetch

Environment (please complete the following information):

• Package Version = 0.1.1
• Python version = 3.8

Additional context
N/A