Issue #173: set the correct mime type of Content-Type in HTTP respose header

ref: http://www.ietf.org/rfc/rfc4627.txt
rfc suggested, the MIME media type for JSON text is application/json instead of
text/html.

issues: https://github.com/kimchi-project/kimchi/issues/173

Signed-off-by: ShaoHe Feng <shaohef@linux.vnet.ibm.com>