Proxy: fixed proxy_set_body with proxy_cache. If the last header evaluation resulted in an empty header, the e.skip flag was set and was not reset when we've switched to evaluation of body_values. This incorrectly resulted in body values being skipped instead of producing some correct body as set by proxy_set_body. Fix is to properly reset the e.skip flag. As the problem only appeared if the last potentially non-empty header happened to be empty, it only manifested itself if proxy_set_body was used with proxy_cache.