ID type support bug causing regression when loading data

Our provider version only supports string id, however when loading data with numeric ids via g.io().read().iterate() the DB didn't error out, but assigned random ids to those vertices with numeric ids, causing regression.

The cause is : https://github.com/apache/tinkerpop/blob/master/gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/Graph.java#L803 

The allowId() method returns false when supportsStringIds() returns true and id is not instance of string. 

Then this method: 

https://github.com/apache/tinkerpop/blob/master/gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/util/Attachable.java#L291

will not pass id into Graph.addVertex() method, making underlying method assign internal ids to the vertex, causing regression between the data we load and the data we store in the DB.