Bug: fiat BOLT-12 offers cannot be paid

Somewhat related to #75

CoinOS is not currently able to parse and pay fiat-denominated offers created manually with CLN.

An offer created like so is correctly parsed and can be paid with CoinOS:

```shell
$ lightning-cli offer 100sat "sat test offer"
```

While this one cannot:

```shell
$ lightning-cli offer 0.10USD "fiat test offer"
```