Logging in lazy sequences may interfere with output of Content-Type: application/edn

[nvlass]

The case addressed in this PR is the following:

1. The request handler response a lazy sequence (i.e. a map of some function function-with-logs on some range).
2. The function-with-logs does some logging (either with prns or with clojure.tools.logging)
3. The sequence is not realised until we read the body, in which case the edn output is mixed with the output of the logging in lazy sequence.

This can be easily avoided if all such responses are realised with a doall, so this pull request might not be a case at all. Forgetting to realise the sequences can, however, result in hard to spot errors.

The solution is not optimal, since it essentially replicates the functionality of pr and pr-on from clojure.core. Perhaps it should also include some tests from clojure.core to ensure that the function remains consistent with pr-str.

Commits:

• Add pr-to-writer in edn.clj, Add test for lazy sequences