[mod_cgi] handle CGI partial response header write

handle CGI partial write of first response header

e.g. gSoap stdsoap2.c might inefficiently write "Status" to response
pipe and lighttpd might read that prior to the backend writing the
subsequent ": " which marks "Status:" as a response header.

x-ref:
  https://redmine.lighttpd.net/boards/2/topics/8028

src/http-header-glue.c

@@ -1096,6 +1096,7 @@ handler_t http_response_parse_headers(server *srv, connection *con, http_respons
 
             last_eol = i;
         }
+    } else if (i == header_len) { /* (no newline yet; partial header line?) */
     } else if (opts->backend == BACKEND_CGI) {
         /* no HTTP headers, but a body (special-case for CGI compat) */
         /* no colon found; does not appear to be HTTP headers */