do not lose value in case it is an integer or a float. Instead convert it to a string.

Eg this returned the default value in the past: `Common::getRequestVar('param', 'string', 'default', array('param' => 5))` which can happen eg in BulkTracking etc. causing many values not to be tracked. Wondering if it breaks anything.
fix and added test
Merge pull request #6720 from piwik/getRequestVarTest

Do not use default value in case value is an integer or a float and a string is requested