Image filter: use "application/json" MIME type for JSON output.

As it is defined by RFC 4627, and allows for various browser tools like
JSONView to display JSON well-formatted.

src/http/modules/ngx_http_image_filter_module.c

diff --git a/src/http/modules/ngx_http_image_filter_module.c b/src/http/modules/ngx_http_image_filter_module.c
index c6c3b74..6d3f47d 100644
--- a/src/http/modules/ngx_http_image_filter_module.c
+++ b/src/http/modules/ngx_http_image_filter_module.c
@@ -567,7 +567,7 @@
     ngx_http_clean_header(r);
 
     r->headers_out.status = NGX_HTTP_OK;
-    ngx_str_set(&r->headers_out.content_type, "text/plain");
+    ngx_str_set(&r->headers_out.content_type, "application/json");
     r->headers_out.content_type_lowcase = NULL;
 
     if (ctx == NULL) {