Add MockHttpServletResponse.getDateHeader

This change adds a new `getDateHeader` method that converts date header
Strings to long values - making tests more readable.

This feature is also documented in the "what's new section" for 4.2.

spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java

@@ -23,6 +23,7 @@ import java.io.OutputStreamWriter;
 import java.io.PrintWriter;
 import java.io.UnsupportedEncodingException;
 import java.io.Writer;
+import java.text.ParseException;
 import java.text.SimpleDateFormat;
 import java.util.ArrayList;
 import java.util.Collection;
@@ -493,6 +494,17 @@ public class MockHttpServletResponse implements HttpServletResponse {
 		setHeaderValue(name, formatDate(value));
 	}
 
+	public long getDateHeader(String name) {
+		SimpleDateFormat dateFormat = new SimpleDateFormat(DATE_FORMAT, Locale.US);
+		dateFormat.setTimeZone(GMT);
+		try {
+			return dateFormat.parse(getHeader(name)).getTime();
+		} catch (ParseException exception) {
+			throw new IllegalArgumentException(
+					"Value for header '" + name + "' is not a valid Date: " + getHeader(name));
+		}
+	}
+
 	@Override
 	public void addDateHeader(String name, long value) {
 		addHeaderValue(name, formatDate(value));