Return empty string instead of `false` on empty response with headers

In case of server response contains some headers but `Content-Length` header has `0` value, then the `false` is returned instead of empty string as a response.

I've fixed similar issue https://github.com/alexandresalome/PHP-Selenium/pull/5 too.

In particular this prevents https://github.com/Behat/SahiClient from working properly, when trying to get an attribute value.