bugfix: lacking the fix from openresty#936 in the C API for lua-resty-core openresty#1031.

Author: moonming

src/ngx_http_lua_headers.c

@@ -1298,13 +1298,30 @@ ngx_http_lua_ffi_get_resp_header(ngx_http_request_t *r,
     ngx_uint_t           i;
     ngx_table_elt_t     *h;
     ngx_list_part_t     *part;
+    ngx_http_lua_ctx_t  *ctx;
+    ngx_int_t            rc;
 
     ngx_http_lua_loc_conf_t     *llcf;
 
     if (r->connection->fd == (ngx_socket_t) -1) {
         return NGX_HTTP_LUA_FFI_BAD_CONTEXT;
     }
 
+    ctx = ngx_http_get_module_ctx(r, ngx_http_lua_module);
+    if (ctx == NULL) {
+        // *errmsg = "no ctx found";
+        return NGX_ERROR;
+    }
+
+    if (!ctx->headers_set) {
+        rc = ngx_http_lua_set_content_type(r);
+        if (rc != NGX_OK) {
+            // *errmsg = "failed to set default content type";
+            return NGX_ERROR;
+        }
+        ctx->headers_set = 1;
+    }
+
     llcf = ngx_http_get_module_loc_conf(r, ngx_http_lua_module);
     if (llcf->transform_underscores_in_resp_headers
         && memchr(key, '_', key_len) != NULL)