Do not require keys to be strings in the python driver

[AtnNn]

This is the current behaviour:

>>> r.expr({r.expr("a"): 1})
rethinkdb.errors.RqlDriverError: Object keys must be strings.
>>> r.object(r.expr("a"), 1).run()
{u'a': 1}

I propose allowing this:

>>> r.expr({r.expr("a"): 1}).run()
{u'a': 1}

Care must be taken when implementing this because OBJECT is not a drop-in replacement for MAKE_OBJ (e.g. #2120)

[coffeemug]

Is there a specific use case for it, or is it just general cleanup? (It would make a difference for scheduling)

[AtnNn]

This would just be a nice shortcut for object.

[mlucy]

If we do this, we should do it in all the drivers. (Also, tagging as REQL_PROPOSAL instead of reql because it isn't 100% obvious to me we should do it. It would involve making the output of r.expr("a") hashable, for example.)

[srh]

We can't do it in all the drivers.

[mlucy]

No, but we can do it in Ruby.