Change the default header date regex

Allow a single "Copyright 20019" form, as well as the standard
"Copyright 2018-2019" range form.

Closes gh-72

Author: philwebb

spring-javaformat/spring-javaformat-checkstyle/src/main/java/io/spring/javaformat/checkstyle/check/SpringHeaderCheck.java

@@ -57,7 +57,7 @@ public class SpringHeaderCheck extends AbstractFileSetCheck {
 	/**
 	 * The default header copyright pattern.
 	 */
-	public static final String DEFAULT_HEADER_COPYRIGHT_PATTERN = "20\\d\\d-20\\d\\d";
+	public static final String DEFAULT_HEADER_COPYRIGHT_PATTERN = "20\\d\\d(-20\\d\\d)?";
 
 	private String headerType = DEFAULT_HEADER_TYPE;
 

spring-javaformat/spring-javaformat-checkstyle/src/test/resources/check/HeaderWithSingleDate.txt

@@ -0,0 +1 @@
++0 errors

spring-javaformat/spring-javaformat-checkstyle/src/test/resources/source/HeaderWithSingleDate.java

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2019-2019 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
+ *
+ *      https://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.
+ */
+
+/**
+ * The header doesn't use a date range.
+ *
+ * @author Phillip Webb
+ */
+public class HeaderWithSingleDate {
+
+}