Don't check for strings and primitives in idmap.

It appears that `Ref::WeakValueMap` is not 100%
correct. There are cases where entries are 
temporarily corrupted; probably while attached
finalizers run.

This is a slight decrease in flexibility in that
strings and primitives cannot maintain referential
integrity when passed by reference to v8, but the
default is to pass them by value anyway, and it
greatly reduces the number of object_ids that the
idmap sees and therefore reduces the chances of 
a recycled object id causing a bad lookup.

While this is not a perfect fix, it does make a
collision very unlikely, and it prevents the 
crashes being seen in #169 

If this pops up again,
we'll have to look at getting a perfectly reliable
weakmap implementation.

benchmarks.rb

@@ -39,11 +39,6 @@ if MODE==:object
     end
 elsif  MODE==:function
     def get(i)
-      test = @cxt['test']
-      unless test
-        $stderr.puts "wtf? #{test.inspect} -> #{@cxt.eval('test')}"
-        $stderr.puts "wff2? #{test.inspect} -> #{@cxt['test']}"
-      end
       @cxt["test"].getObject(i)
     end
 elsif  MODE==:eval

lib/v8/conversion/indentity.rb

@@ -11,6 +11,7 @@ class V8::Conversion
     end
 
     def to_v8(ruby_object)
+      return super(ruby_object) if ruby_object.is_a?(String) || ruby_object.is_a?(Primitive)
       rb_idmap[ruby_object.object_id] || super(ruby_object)
     end