Address issues with current PHP 5.6 RC

This pull addresses issues found running PHPUnit on the PHP 5.6 RC 2 release.

1) JFormTest calling non-static methods of the test inspector class it uses statically (no production code/changes involved)
2) JFormTest has an inherent dependency on a database connection because of a chain of method calls within it causing the test class to fail when run in isolation (no production changes involved)
3) In PHP 5.6, the `iconv_set_encoding()` function and the settings it controls are deprecated in favor of a `default_charset` setting, see https://wiki.php.net/rfc/default_encoding for more info.  The deprecation notices caused test errors on PHP 5.6.

Unit testing against PHP 5.4 and 5.6 are still good for me on my local system with these changes.  The change in JString probably just needs someone to plug it into a site to make sure the settings still set correctly.