DuckPAN/Web: Default to DDG's endpoint if API key is missing.

The lack of publicly available API keys have prevented open-source developers from correctly fixing / testing Spice instant answers without DDG's help. The developer, especially new ones, will run into one of these two problems:

1. She has to register for an API key for an instant answer she has to fix.
2. She cannot register for an API key at all (because the API key was exclusively given to DDG).

I think this fix will make it easier for people to test their fixes and changes to JavaScript.

CC @crazedpsyc @moollaza