fixes parsing deep nested resources from XML

This patch fix a bug on ActiveResource::Base#find_or_create_resource_for which avoids to parse correctly deep nested resources from XML.

When parsing deep nested resources from a XML representation, ActiveResource builds nested classes (resources) starting from root resource. The problem happens when the XML has an array of resources more than second level of nesting because it tries to set the same resource more than one time on the same base resource. The bug shows shows the warning: "warning: already initialized constant [ResourceClassName]"; also parsing fails.