Ger rid of reflection warnings

[markokocic]

While trying to compile my own project with warn-on-reflection in Clojure 1.3.0 I got the following warnings in ring code.

Reflection warning, ring/util/response.clj:50 - reference to field isDirectory can't be resolved.
Reflection warning, ring/util/response.clj:52 - reference to field exists can't be resolved.
Reflection warning, ring/util/servlet.clj:64 - call to setContentType can't be resolved.
Reflection warning, ring/util/servlet.clj:78 - call to setCharacterEncoding can't be resolved.
Reflection warning, ring/middleware/multipart_params/byte_array.clj:14 - call to toByteArray can't be resolved.
Reflection warning, ring/middleware/resource.clj:15 - call to substring can't be resolved.
Reflection warning, ring/middleware/session/cookie.clj:81 - reference to field length can't be resolved.
Reflection warning, ring/middleware/session/cookie.clj:81 - reference to field length can't be resolved.
Reflection warning, ring/middleware/session/cookie.clj:85 - reference to field getBytes can't be resolved.
Reflection warning, ring/middleware/session/cookie.clj:86 - reference to field getBytes can't be resolved.

It is possible that there are more.