Add test for whitespace-only lines

Author: nwoythal

spec/unit/puppet-lint/plugins/check_whitespace/trailing_whitespace_spec.rb

@@ -105,5 +105,17 @@
         expect(manifest).to eq(fixed)
       end
     end
+
+    context 'empty line with nothing but whitespace' do
+      let(:code) { " \n " }
+
+      it 'only detects a single problem' do
+        expect(problems).to have(2).problem
+      end
+
+      it 'fixes the manifest' do
+        expect(manifest).to eq("\n")
+      end
+    end
   end
 end