Two bug fixes for process.mixin

Bug #1 occurred when trying to use process.mixin on a function and
produced a fatal exception.

Bug #2 occurred when trying to do a deep merge with an object containing
one or more objects with a nodeType property. In those cases the deep
copy for this part of the object was not performed and a shallow one was
performed instead.

Both of these bugs were artifacts of the jQuery.extend port.