Add locking to AbstractRequestExpectationManager

This commit ensures that verifying a request, which includes finding
and updating expectations, is done synchronously to avoid concurrent
modification exceptions.

Technically SimpleRequestExpectationManager is not even expected to see
concurrent requests by definition but with
UnorderedRequestExpectationManager it can happen.

Issue: SPR-15029

spring-test/src/main/java/org/springframework/test/web/client/AbstractRequestExpectationManager.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -46,6 +46,8 @@ public abstract class AbstractRequestExpectationManager implements RequestExpect
 
 	private final List<ClientHttpRequest> requests = new LinkedList<ClientHttpRequest>();
 
+	private final Object lock = new Object();
+
 
 	protected List<RequestExpectation> getExpectations() {
 		return this.expectations;
@@ -66,12 +68,14 @@ public abstract class AbstractRequestExpectationManager implements RequestExpect
 
 	@Override
 	public ClientHttpResponse validateRequest(ClientHttpRequest request) throws IOException {
-		if (getRequests().isEmpty()) {
-			afterExpectationsDeclared();
+		synchronized (this.lock) {
+			if (getRequests().isEmpty()) {
+				afterExpectationsDeclared();
+			}
+			ClientHttpResponse response = validateRequestInternal(request);
+			getRequests().add(request);
+			return response;
 		}
-		ClientHttpResponse response = validateRequestInternal(request);
-		getRequests().add(request);
-		return response;
 	}
 
 	/**