title: Saving RelationshipEntity not working
body: <p>I am new to Neo4J and working with Spring data repository. Following is the domain definition</p>
  
  <pre><code>@NodeEntity
  public class Actor {
      Long id;
      private Set&lt;Role&gt; roles;
  }
  
  @RelationshipEntity(type="PLAYED_IN")
  public class Role {
      @GraphId   private Long relationshipId;
      @Property  private String title;
      @StartNode private Actor actor;
      @EndNode   private Movie movie;
  }
  
  @NodeEntity
  public class Movie {
      private Long id;
      private String title;
  }
  </code></pre>
  
  <p>And have <code>GraphRepository</code> defined for each entity class 
  Following code does not save the RelationshipEntity</p>
  
  <pre><code>Actor actor = new Actor("actorName");
  actor = actorRepository.save(actor);
  
  Movie movie = new Movie("movieTitle");
  movie = movieRepository.save(movie);
  
  Role role = new Role(actor, movie, "roleTitle");
  role = roleRepository.save(role);
  </code></pre>
  
  <p>Do I have to annotate the <code>roles</code> variable in <code>Actor</code> class? <br>
  Do I have to populate <code>roles</code> collection before saving <code>Actor</code>? If I do so then the properties on <code>Role</code> are not saved.</p>