Fix APICallMixin url_args handling

[30350n]

The current implementation of APICallMixin.api_call handles url_args very weirdly. This improves the behavior, so that non single character string args will be handled correctly.

Before:

>>> api_build_url_args({'a': 'abc123'})
'?a=a,b,c,1,2,3'

After:

>>> api_build_url_args({'a': 'abc123'})
'?a=abc123'

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[SchrodingersGat]

Nice catch :)