Add support for zero-amount Lightning invoice payments

[bitwalt]

• Add test case for zero-amount invoice payment in payment module
• Fix panic in send_payment function when handling zero-amount invoices
• Remove redundant unwrap() that was shadowing properly-calculated amt_msat
• Zero-amount invoices now properly accept explicit payment amounts
• Format code with cargo fmt

Fixes #88

[zoedberg]

Thanks for finding and fixing this.

Please make the following changes:

• split the commit in 2, one with just the test and the second one with the fix. This allows to see the test successfully reproduces the bug
• to avoid very long commit names put an empty line between the short and long description otherwise the git CLI will treat everything as a short description
• the test is using the same directory of the zero_amount_invoice test, so one test will override the data of the other. I think that instead of adding a new test you can just add the payment part to the already existing test

[bitwalt]

Should be done! Let me know if there's something else to change

[zoedberg]

LGTM, thank you!