Passing floats in arguments to past_event_settings results in errors on display

In PastEventArgument->ensureType the PHP function gettype() is called to get the type of the argument. The PHP docs says that if a float is passed to that function, the string "double" is returned for historical (and quite annoying) reasons: http://dk1.php.net/manual/en/function.gettype.php

If floats are passed in as arguments, the display of them later in admin/reports/past/xx fails with a "Cannot access empty property in ..../past_db.entity.inc on line 274" because the scalar test failed and we attempt to make the data into an object.

Patch correctes "float" to "double".