Simplified string-contains?

Replaced Java .indexOf string method with .contains for clarity, added
tests to cover the function.
Extended the test for dissoc-if-nil

Added a test case for multiple keys, only some of which are nil.
Minor changes to string-contains? and string-contains?-test
Merge pull request #745 from holguinj/master

Clojure cleanup in string-contains?, add/extend tests for string-contains? and dissoc-if-nil.