GH-312 Add default constructor for TupleBuilder

Resolves #312

The TupleBuilder in XD provided a zero arg constructor. This restores backward compatibility by adding it back.

spring-cloud-stream-tuple/src/main/java/org/springframework/cloud/stream/tuple/TupleBuilder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2013-2015 the original author or authors.
+ * Copyright 2013-2016 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.
@@ -84,6 +84,13 @@ public class TupleBuilder {
 		return new TupleBuilder(false);
 	}
 
+	/**
+	 * Creates a builder that will create immutable Tuples.
+	 */
+	public TupleBuilder() {
+		this(false);
+	}
+
 	private TupleBuilder(boolean mutable) {
 		this.mutable = mutable;
 	}