This fixes an issue where attempting to use "!=" in a condition with an array with only a single element generates invalid SQL.

Example:
$condition['Model.id !='] = array(1, 2); //Generates Model.id NOT IN (1, 2) as expected
$condition['Model.id !='] = array(1); //Generates Model.id != = (1) which is invalid SQL

Patch will cause the above to generate Model.id != (1);

This an implimentation of @markstory's suggestion on PR 1232
Add unit test
Merge pull request #1308 from Jippi/hotfix/sql-bug-with-not-in-with-one-key

Hotfix/sql bug with NOT in with one key