Add missing tests for statement_count functions

test/unit/dataset_test.exs

@@ -1616,6 +1616,11 @@ defmodule RDF.DatasetTest do
     assert Enum.count(dataset.graphs) == 1
   end
 
+  test "statement_count/1" do
+    assert Dataset.statement_count(dataset()) == 0
+    assert Dataset.statement_count(Dataset.new(statement())) == 1
+  end
+
   describe "include?/3" do
     test "valid cases" do
       dataset =

test/unit/description_test.exs

@@ -787,6 +787,11 @@ defmodule RDF.DescriptionTest do
     assert Enum.count(desc.predications) == 1
   end
 
+  test "statement_count/1" do
+    assert Description.statement_count(Description.new(EX.S)) == 0
+    assert Description.statement_count(description({EX.p(), EX.O})) == 1
+  end
+
   describe "include?/3" do
     test "valid cases" do
       desc =

test/unit/graph_test.exs

@@ -1283,6 +1283,11 @@ defmodule RDF.GraphTest do
     assert Enum.count(graph.descriptions) == 1
   end
 
+  test "statement_count/1" do
+    assert Graph.statement_count(graph()) == 0
+    assert Graph.statement_count(Graph.new(statement())) == 1
+  end
+
   describe "include?/3" do
     test "valid cases" do
       graph =