Support arrays in GraphSON

JanusGraph supports arrays of primitive types for property types:
```groovy
mgmt.makePropertyKey('vec').dataType(float[]).cardinality(Cardinality.SINGLE).make()
```

Unfortunately, this feature cannot be used with GraphSON right now as we don't have serialization support for arrays. Instead, the array is serialized as a list by the driver which leads to an error on the server side:
```
Expected: class [F, found: class java.util.ArrayList
```

(More information with a concrete example on [gremlin-users](https://groups.google.com/d/msg/gremlin-users/3OyXb1x0s4Q/pm93joFiBAAJ).)

The issue names GraphSON serialization but no source file or test entry point. Start by locating the GraphSON serialization code and its tests, then reproduce the primitive-array example and the ArrayList type error. Done means primitive arrays such as float[] can be serialized and accepted by the server without being treated as lists.

Written by the indexing model from the issue text.