Support trailing comment in DTD declaration in XML config

Prior to this commit, Spring failed to determine that an XML config file
was DTD-based if the DTD declaration was followed by a comment.

This commit fixes this by modifying the consumeCommentTokens(String)
algorithm in XmlValidationModeDetector so that both leading and trailing
comments are properly consumed without losing any XML content.

Closes gh-23605

spring-core/src/test/java/org/springframework/util/xml/XmlValidationModeDetectorTests.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2002-2019 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.
+ */
+
+package org.springframework.util.xml;
+
+import java.io.InputStream;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+import static org.springframework.util.xml.XmlValidationModeDetector.VALIDATION_DTD;
+
+/**
+ * Unit tests for {@link XmlValidationModeDetector}.
+ *
+ * @author Sam Brannen
+ * @since 5.1.10
+ */
+public class XmlValidationModeDetectorTests {
+
+	private final XmlValidationModeDetector xmlValidationModeDetector = new XmlValidationModeDetector();
+
+
+	@Test
+	public void dtdWithTrailingComment() throws Exception {
+		dtdDetection("dtdWithTrailingComment.xml");
+	}
+
+	@Test
+	public void dtdWithLeadingComment() throws Exception {
+		dtdDetection("dtdWithLeadingComment.xml");
+	}
+
+	@Test
+	public void dtdWithCommentOnNextLine() throws Exception {
+		dtdDetection("dtdWithCommentOnNextLine.xml");
+	}
+
+	@Test
+	public void dtdWithMultipleComments() throws Exception {
+		dtdDetection("dtdWithMultipleComments.xml");
+	}
+
+	private void dtdDetection(String fileName) throws Exception {
+		InputStream inputStream = getClass().getResourceAsStream(fileName);
+		assertEquals(VALIDATION_DTD, xmlValidationModeDetector.detectValidationMode(inputStream));
+	}
+
+}