Failing test case for #89
Content-type interceptor that doesn't inject octet-stream

Unlike Ring's content-type-response function, ours does not inject
"application/octet-stream" if no existing or URI-based MIME type can be
found. This allows the current default-content-type of "text/plain" to
crop up.
Test that stresses content-type-response not meddling
Correct name of plaintext-body test
Merge pull request #165 from pedestal/default-content-type

Return default content type to text/plain. Fixes #89.