Backend shouldn't default to auto if non-disabled engines are provided

[tallm0cha]

Currently google and bing are disabled.
If one puts them in the backend list, ddgs uses auto instead of looking through other engines provided:
results = list(ddgs.text(query, max_results=max_results, backend="brave, yahoo, duckduckgo, google, bing"))
Result:
ddgs.ddgs - WARNING - KeyError('google') - backend is not exist or disabled. Available: brave, duckduckgo, grokipedia, mojeek, wikipedia, yahoo, yandex. Using 'auto'

Ideal behavior: ddgs should have skipped google and bing and used the other engines.

[deedy5]

Fixed in #456