Update language tag validation to what BCP47 requires, not RDF, which is too libreral.

Author: gkellogg

lib/rdf/model/literal.rb

@@ -360,7 +360,7 @@ def has_datatype?
     # @return [Boolean] `true` or `false`
     # @since  0.2.1
     def valid?
-      return false if language? && language.to_s !~ /^[a-zA-Z]+(-[a-zA-Z0-9]+)*$/
+      return false if language? && language.to_s !~ /^[a-zA-Z]{1,8}(-[a-zA-Z0-9]{1,8})*$/
       return false if datatype? && datatype.invalid?
       grammar = self.class.const_get(:GRAMMAR) rescue nil
       grammar.nil? || value.match?(grammar)