Fix session bug
Use Labellable#labels_to_stream
[#41182989] Fixed core_resource

Changed labels_to_stream method name to 
labellable_to_stream, because this method
has been overridden by labellable_resource class
Also fixed: add labels hash to resource, if there
is at least an existing label for that resource.
[#41182989] Added a test for labellable with labels.
[#41182989] Fixed label spec
[#41182989] Fixed flowcell, tube, plate spec
[#41182989] Fixed last_session related problems