Directly updating the GlobalContextRegistry doesn't clear dependents

[jturkel]

To reproduce:

Organization.current = Organization.find(1)
# => #<Organization:0x000000011c9475d8 id: 1, ...>

# PropertyCache is dependent_on Organization
PropertyCache.current
#=> #<PropertyCache:0x000000010b2b3a48>

# Directly update the registry using [], set, with_merged_registry, or merge!
RailsMultitenant[:organization_id] = 2

Organization.current_id
# => 2

# RailsMultitenant[:organization_obj] still points to old organization
Organization.current
# => #<Organization:0x000000011c9475d8 id: 1, ...>

# Context objects dependent on organization aren't cleared out
PropertyCache.current
#=> #<PropertyCache:0x000000010b2b3a48>