Literal languages are lost on repeated `#<<` calls

Created by: no-reply

We lose language values when setting using existing values. This affects #<<, and #set calls when a Relation is the argument.

require 'rdf/vocab'

class Book
  include ActiveTriples::RDFSource  
  property :title, predicate: RDF::Vocab::DC.title  
end  

book = Book.new

book.title << RDF::Literal("Moomin", language: :en)
book.title << RDF::Literal("Mummi",  language: :fi)

book.dump :ttl
# => "\n [ <http://purl.org/dc/terms/title> \"Moomin\",\n      \"Mummi\"@fi] .\n"⁠