Fix: IE Class Retention (Issue #937)

In adaptContainerCssClass, variable "c" in a string in FF, but an object in IE.  We've adapted the function so that if it is an object and has a length, then return it as a string, otherwise to return the object (null/undefined?)

This should fix https://github.com/ivaynberg/select2/issues/937
Credit to @frntz for also identifying the origin of the issue