Fix #1119: Stop overwrite of first header on mruby call to env.req.set_header(..)

src/shrpx_mruby_module_request.cc

@@ -246,8 +246,9 @@ mrb_value request_mod_header(mrb_state *mrb, mrb_value self, bool repl) {
         continue;
       }
       if (i != p) {
-        headers[p++] = std::move(kv);
+        headers[p] = std::move(kv);
       }
+      ++p;
     }
     headers.resize(p);
   }