When generating an invoice for less than 20 cents, client does not return BTC addresses

[tonydavis629]

invoice_response = client.create_invoice({"price": 1.0, "currency": "USD"})

Returns a normal response.

invoice_response = client.create_invoice({"price": .19, "currency": "USD"})

Returns a response without a BTC address (but does include other currencies).

invoice_response = client.create_invoice({"price": .20, "currency": "USD"})

Also returns a normal response.