[Bug] [Rest API]: Literal error when a comma-separated list of countries is used.

[Danglewood]

Entering a comma-separated list of countries to the CPI REST API endpoint creates a "literal_error".

The Swagger docs show the url as each item in the list being an independent declaration of the "countries" param:

...economy/cpi?provider=fred&countries=australia&countries=austria&countries=belgium&

The URL should work in the same way other multi-ticker inputs operates:

economy/cpi?provider=fred&countries=austria,belgium,australia