Make it easier to write a preprocessor that only changes req or resp

The commit introduces OperationPreprocessorAdapter, an abstract
implementation of OperationProcessor that returns to request and
response as-is. OperationPreprocessorAdapter is intended to be
subclassed by OperationProcessor implementations that only modify
the request or the response. Implementations the modify both should
continue to implement OperationPreprocess directly.

Closes gh-154

spring-restdocs-core/src/main/java/org/springframework/restdocs/operation/preprocess/OperationPreprocessorAdapter.java

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2012-2016 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.restdocs.operation.preprocess;
+
+import org.springframework.restdocs.operation.OperationRequest;
+import org.springframework.restdocs.operation.OperationResponse;
+
+/**
+ * An implementation of {@link OperationPreprocessor} that returns the request and
+ * response as-is. To be subclasses by preprocessor implementations that only modify the
+ * request or the response.
+ *
+ * @author Andy Wilkinson
+ * @since 1.1.0
+ */
+public abstract class OperationPreprocessorAdapter implements OperationPreprocessor {
+
+	/**
+	 * Returns the given {@code request} as-is.
+	 *
+	 * @param request the request
+	 * @return the unmodified request
+	 */
+	@Override
+	public OperationRequest preprocess(OperationRequest request) {
+		return request;
+	}
+
+	/**
+	 * Returns the given {@code response} as-is.
+	 *
+	 * @param response the response
+	 * @return the unmodified response
+	 */
+	@Override
+	public OperationResponse preprocess(OperationResponse response) {
+		return response;
+	}
+
+}