Finalizing inheritance for fluid-object

[bmiww]

Fixes an error where the first time the fluid-object is being instantiated - its class inheritance is not yet finalized.
Affects cases where a the with-decoder-simple-clos-semantics is used without an explicit class (a default fluid-object class is used).

Fixes 4 of the test cases:

TEST-ENCODE-JSON-CLOS-MAX-PACKAGE in JSON []:
TEST-ENCODE-JSON-CLOS in JSON []:
JSON-OBJECT-WITH-PROTOTYPE in JSON []:
JSON-OBJECT in JSON []:

I've only tested this with SBCL so far.
Haven't tested it in too much effort yet, but as far as i understand - when new classes/slots would appear - the class would go through reinitialization anyways, so I'm not expecting this to have negative effects.
(not an expert though :) )