Fix RDF.LangString to produce invalid literals when invalid languages given

CHANGELOG.md

@@ -30,6 +30,8 @@ This project adheres to [Semantic Versioning](http://semver.org/) and
 
 - `RDF.DateTime`: '24:00:00' is a valid time in a xsd:dateTime; the dateTime 
   value so represented is the first instant of the following day
+- `RDF.LangString`: non-strings or the empty string as language produce invalid
+  literals
 
 
 [Compare v0.4.1...HEAD](https://github.com/marcelotto/rdf-ex/compare/v0.4.1...HEAD)

lib/rdf/datatypes/lang_string.ex

@@ -6,7 +6,8 @@ defmodule RDF.LangString do
   use RDF.Datatype, id: RDF.uri("http://www.w3.org/1999/02/22-rdf-syntax-ns#langString")
 
 
-  def build_literal(value, lexical, %{language: language} = opts) do
+  def build_literal(value, lexical, %{language: language} = opts)
+      when is_binary(language) and language != "" do
     %Literal{super(value, lexical, opts) | language: String.downcase(language)}
   end
 

test/unit/datatypes/lang_string_test.exs

@@ -42,12 +42,26 @@ defmodule RDF.LangStringTest do
       end
     end
 
-    test "without a language it warns and produces an invalid literal" do
+    test "without a language it produces an invalid literal" do
       Enum.each @valid, fn {input, _} ->
         assert %Literal{} = literal = LangString.new(input)
         refute Literal.valid?(literal)
       end
     end
+
+    test "with nil as a language it produces an invalid literal" do
+      Enum.each @valid, fn {input, _} ->
+        assert %Literal{} = literal = LangString.new(input, language: nil)
+        refute Literal.valid?(literal)
+      end
+    end
+
+    test "with the empty string as a language it produces an invalid literal" do
+      Enum.each @valid, fn {input, _} ->
+        assert %Literal{} = literal = LangString.new(input, language: "")
+        refute Literal.valid?(literal)
+      end
+    end
   end
 
 
@@ -68,6 +82,18 @@ defmodule RDF.LangStringTest do
         assert_raise ArgumentError, fn -> LangString.new!(input) end
       end
     end
+
+    test "with nil as a language it raises an error" do
+      Enum.each @valid, fn {input, _} ->
+        assert_raise ArgumentError, fn -> LangString.new!(input, language: nil) end
+      end
+    end
+
+    test "with the empty string as a language it raises an error" do
+      Enum.each @valid, fn {input, _} ->
+        assert_raise ArgumentError, fn -> LangString.new!(input, language: "") end
+      end
+    end
   end