Fix duplicated Content-Type for show/update functions
Fix duplicated Content-Type for show/update functions

When a show/update function returned data it added a
"Content-Type: application/json" header by itself. Then
couch_httpd added some default headers (including a yet
another "application/json" one).
This diff will add default chttpd headers only when
same headers are not yet present in the response.

This closes #55

COUCHDB-1876

Signed-off-by: Alexander Shorin <287dfd97d2274fe201ae7191a244ba25e475e13a@apache.org>

fkaempfer/couchdb,fkaempfer/couchdb,smithsz/couchdb-couch,fkaempfer/couchdb,apache/couchdb-couch,smithsz/couchdb-couch,fkaempfer/couchdb,smithsz/couchdb-couch,cloudant/couchdb-couch,cloudant/couchdb-couch,apache/couchdb-couch,fkaempfer/couchdb,cloudant/couchdb-couch,fkaempfer/couchdb,apache/couchdb-couch