Handle null header value property

When using an Apache Http components based infrastructure, a null header
value is handled as the empty string. The exact same infrastructure using
HttpURLConnection generates a header with no colon. This is actually not
proper HTTP and some components fail to read such request.

We now make sure to call HttpURLConnection#addRequestProperty with the
empty String for a null header value.

Issue: SPR-13225

spring-web/src/test/java/org/springframework/http/client/SimpleClientHttpRequestFactoryTests.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2002-2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.http.client;
+
+import java.net.HttpURLConnection;
+
+import org.junit.Test;
+
+import org.springframework.http.HttpHeaders;
+
+import static org.mockito.Mockito.*;
+
+/**
+ * @author Stephane Nicoll
+ */
+public class SimpleClientHttpRequestFactoryTests {
+
+	@Test // SPR-13225
+	public void headerWithNullValue() {
+		HttpURLConnection urlConnection = mock(HttpURLConnection.class);
+		HttpHeaders headers = new HttpHeaders();
+		headers.set("foo", null);
+		SimpleBufferingClientHttpRequest.addHeaders(urlConnection, headers);
+		verify(urlConnection, times(1)).addRequestProperty("foo", "");
+	}
+
+}