bugfix: missing the content-type header introduced by commit de1b896.

Author: zhuizhuhaomeng

src/ngx_http_lua_util.c

@@ -535,11 +535,13 @@ ngx_http_lua_send_header_if_needed(ngx_http_request_t *r,
             r->headers_out.status = NGX_HTTP_OK;
         }
 
-        if (!ctx->headers_set) {
-            if (ngx_http_lua_set_content_type(r, ctx) != NGX_OK) {
-                return NGX_ERROR;
-            }
+        if (!ctx->mime_set
+            && ngx_http_lua_set_content_type(r, ctx) != NGX_OK)
+        {
+            return NGX_ERROR;
+        }
 
+        if (!ctx->headers_set) {
             ngx_http_clear_content_length(r);
             ngx_http_clear_accept_ranges(r);
         }