Fix RuboCop offenses in tests

test/test_document.rb

@@ -39,7 +39,7 @@ class TestDocument < JekyllUnitTest
     end
 
     should "exist" do
-      refute @document.nil?
+      refute_nil @document
     end
 
     should "know its relative path" do

test/test_utils.rb

@@ -130,7 +130,7 @@ class TestUtils < JekyllUnitTest
 
   context "The \`Utils.slugify\` method" do
     should "return nil if passed nil" do
-      assert Utils.slugify(nil).nil?
+      assert_nil Utils.slugify(nil)
     rescue NoMethodError
       assert false, "Threw NoMethodError"
     end