Headers map is not ring compliant - may contains Integers

[ekimber]

The "content-length" element is an Integer. But it would be nice if the :headers element was compatible with the ring schema because other libraries depend on this. Quote from the ring wiki:

:headers A Clojure map of HTTP header names to header values. These values may either be strings, in which case one name/value header will be sent in the HTTP response, or a collection of strings, in which case a name/value header will be sent for each value.