Consistent available() support in DelegatingServletInputStream

Issue: SPR-16416

spring-web/src/test/java/org/springframework/mock/web/test/DelegatingServletInputStream.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -66,6 +66,11 @@ public class DelegatingServletInputStream extends ServletInputStream {
 		return data;
 	}
 
+	@Override
+	public int available() throws IOException {
+		return this.sourceStream.available();
+	}
+
 	@Override
 	public void close() throws IOException {
 		super.close();