title: Enum conversion to String not working on @Indexed unique field
body: <p>I'm a Neo4j/Spring-data newbie so apologies if this is something obvious but I looked here and there and can't quite figure out if it's a bug or feature.
  I'm using SDN 3.1.0 and Neo4j 2.0.4, running in memory for now (for testing).</p>
  
  <p>I have a super simple POJO that I try to save into Neo4j using SDN. It looks like so:</p>
  
  <pre><code>@NodeEntity
  public class Weekday {
  
  @GraphId
  private Long id;
  
  @Indexed(unique = true)
  public DayOfWeek weekdayCode;
  }
  </code></pre>
  
  <p>Everything works beautifully when I make it non-uniquely indexed, or not indexed at all. It works fine with unique constraint when I make it a String as well. (Well, sort of, I'm aware that it doesn't throw an exception but silently updates existing one - this is not perfect but I found JIRA issue related to that). Unfortunately the moment I try to save it as enum with unique constraint I get an exception:</p>
  
  <pre><code>org.springframework.dao.InvalidDataAccessResourceUsageException: Error executing statement MERGE (n:`Weekday` {`weekdayCode`: {value}}) ON CREATE SET n={props}  return n; nested exception is org.springframework.dao.InvalidDataAccessResourceUsageException: Error executing statement MERGE (n:`Weekday` {`weekdayCode`: {value}}) ON CREATE SET n={props}  return n; nested exception is java.lang.IllegalArgumentException: [MONDAY:java.time.DayOfWeek] is not a supported property value
  at org.springframework.data.neo4j.support.query.CypherQueryEngineImpl.query(CypherQueryEngineImpl.java:61)
  at org.springframework.data.neo4j.support.schema.SchemaIndexProvider.merge(SchemaIndexProvider.java:114)
  at [...]
  Caused by: org.springframework.dao.InvalidDataAccessResourceUsageException: Error executing statement MERGE (n:`Weekday` {`weekdayCode`: {value}}) ON CREATE SET n={props}  return n; nested exception is java.lang.IllegalArgumentException: [MONDAY:java.time.DayOfWeek] is not a supported property value
  at org.springframework.data.neo4j.support.query.CypherQueryEngineImpl.parseAndExecuteQuery(CypherQueryEngineImpl.java:72)
  at org.springframework.data.neo4j.support.query.CypherQueryEngineImpl.query(CypherQueryEngineImpl.java:58)
  ... 63 more
  Caused by: java.lang.IllegalArgumentException: [MONDAY:java.time.DayOfWeek] is not a supported property value
  at org.neo4j.kernel.api.properties.PropertyConversion.convertProperty(PropertyConversion.java:107)
  at org.neo4j.kernel.api.properties.Property.property(Property.java:51)
  at [...]
  </code></pre>
  
  <p>This, as far as I can see, is because the unique field is put into a map "props", and the contents of the map is not automatically converted so it sends an enum to Neo4j, which obviously it doesn't like.</p>
  
  <ol>
  <li>Is this expected or should I raise a bug with SDN?</li>
  <li>If that's expected behaviour, do I have any alternatives other than making the field a String?</li>
  </ol>