Calling RdfDatstream#deserialize unexpectedly returns RubydoraRepository

[jcoyne]

When the object is persisted, and the datastream is empty.

       expected RDF::Statement, Hash, or Array, but got #<Rubydora::Repository:0x007fb7b10bcaf0 @config={:user=>"fedoraAdmin", :password=>"fedoraAdmin", :url=>"http://127.0.0.1:8983/fedora-test"}, @api=#<Rubydora::Fc3Service:0x007fb7b1145c60 @config={:user=>"fedoraAdmin", :password=>"fedoraAdmin", :url=>"http://127.0.0.1:8983/fedora-test"}, @config_hash=-1452056331661277558, @client=#<RestClient::Resource:0x007fb7b1145b20 @url="http://127.0.0.1:8983/fedora-test", @block=nil, @options={:user=>"fedoraAdmin", :password=>"fedoraAdmin", :open_timeout=>nil}>>, @version=3.7>
     # /Users/justin/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/rdf-1.1.5.1/lib/rdf/model/statement.rb:41:in `from'
     # /Users/justin/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/rdf-1.1.5.1/lib/rdf/mixin/writable.rb:39:in `<<'
     # /Users/justin/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/rdf-1.1.5.1/lib/rdf/mixin/mutable.rb:71:in `<<'
     # /Users/justin/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/active-fedora-7.1.0/lib/active_fedora/rdf/rdf_datastream.rb:72:in `resource'