Replace use of 'read-string' in test with clojure.edn

This particular usage of 'read-string' does not expose the system to
arbitrary code execution from external sources, but it is a good habit
to avoid invoking the Clojure reader when a simpler, more secure
alternative is available.
Merge pull request #40 from pedestal/remove-read-string

Replace use of 'read-string' in test with clojure.edn