Various additions & fixes for accept-language bug.

Bug is #20205 on PEAR at https://pear.php.net/bugs/bug.php?id=20205

 * Fixed parameter name (accept-language v accept_language) in Nominatim query.
 * Added support for setting accept-language config value.
 * Unit tests for Config and Nominatim classes.

Thanks to Alex Sautner for making me aware of this problem.