Automatically autowire a bean with one constructor

Previously, if a managed bean had only one non-default constructor, we
should still annotate it with `@Autowired` to properly use constructor
injection. Not doing so resulted in an error as the container was
trying to call the default (non-existing) constructor.

This commit updates this behaviour to automatically applyed the
autowiring semantic to any bean that has only one constructor. As
before, if more than one constructor is defined, `@Autowired` must be
specified to teach the container the constructor it has to use.

Issue: SPR-12278

spring-beans/src/main/java/org/springframework/beans/factory/annotation/AutowiredAnnotationBeanPostProcessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 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.
@@ -107,6 +107,7 @@ import org.springframework.util.StringUtils;
  *
  * @author Juergen Hoeller
  * @author Mark Fisher
+ * @author Stephane Nicoll
  * @since 2.5
  * @see #setAutowiredAnnotationType
  * @see Autowired
@@ -312,6 +313,9 @@ public class AutowiredAnnotationBeanPostProcessor extends InstantiationAwareBean
 						}
 						candidateConstructors = candidates.toArray(new Constructor<?>[candidates.size()]);
 					}
+					else if (rawCandidates.length == 1 && rawCandidates[0].getParameterTypes().length > 0) {
+						candidateConstructors = new Constructor<?>[] {rawCandidates[0]};
+					}
 					else {
 						candidateConstructors = new Constructor<?>[0];
 					}