Fix rule and check.

[terazus]

The response[0] in core was setting what was supposed to be a list to a single string from the list instead. The check for whether a ontology source was in the list was nested beneath another check instead of being its own check.

[coveralls]

coverage: 81.282% (-0.02%) from 81.298%
when pulling 9a5fb86 on ptth222:ontology-source-check-fix
into dbfa307 on ISA-tools:issue-511.

[ptth222]

Here are screenshots of the new missing lines. I think the missing lines in rules/core.py are because this fix stopped exceptions from trying to access the 0th element in an empty list.

The new missing line in rules_30xx.py is from breaking the 2 tests into 2 separate if statements. The same check still isn't being tested just like before, there is just an extra line there now.