DATAMONGO-1050 - Explicitly annotated Field should not be considered Id.

We changed the id resolution to skip properties having an explicit name set via @Field unless they are marked with @Id. This means that

@Field(“id”) String id;

will be stored as “id” within mongodb. Prior to this change the fieldname would have been changed to “_id”.
Added tests to ensure proper field mapping for various "id" field variants.

Original pull request: #225.