Fixed application/json response header issues

[gary-archer]

For error responses, the application/json response header was written incorrectly.
Fix this by moving the setting of the header before the ngx_http_send_header() call.

Also, for local macOS and local Docker testing, JSON responses were incorrect:

• There was a missing } in the test JSON response body.
• The content-type directive in the location block should be set according to this answer.