guard against nil contentType

- As of ClojureScript
  0.0-2280 (https://github.com/clojure/clojurescript/commit/774d4588581f6d29a1b6b2555171d3f3d36c2c83),
  `re-find` will throw a TypeError for nil. If the caller of
  `jayq.core/preprocess-request` does not pass in a `contentType`, we
  should not be associng `:data` into the request.