Handle uninitialized constant JSON::GenericObject at ruby/ruby.

```
  1) Error:
JSONGenericObjectTest#test_from_hash:
Test::Unit::ProxyError: uninitialized constant JSON::GenericObject
    /Users/hsbt/Documents/github.com/ruby/ruby/.ext/common/json/common.rb:1006:in 'JSON.const_missing'
    /Users/hsbt/Documents/github.com/ruby/ruby/test/json/json_generic_object_test.rb:8:in 'JSONGenericObjectTest#setup'
```

Author: hsbt

test/json/json_generic_object_test.rb

@@ -9,6 +9,8 @@ def setup
     else
       omit("JSON::GenericObject is not available")
     end
+  rescue NameError
+    omit("JSON::GenericObject is not available")
   end
 
   def test_attributes