Allow dump_object to be used with primary keys that are longs

I needed to use the dump_object command to generate fixtures for a model with primary keys that are longs (i.e. not ints or strings).  For example dump_object myModel 437L.  With the existing code, that would be converted to the string "437L", and that breaks when the query is executed.  

I acknowledge that this solution won't work for string primary keys that look like python long literals (e.g. if you have a primary key that is a string equal to something like "437L"), but I feel like that would be an extremely rare case and that the use of longs in general as primary keys is a common enough scenario such that it warrants this change.

Let me know what you think.