bug: fix query representation to string

[liranbg]

Reason for the change
If applicable, link the related issue/bug report or write down in few sentences the motivation.

Trying to build a query dynamically and log it

Description
Seems like there was a mass change from st to statement during rethinkdb 2.1->2.4 but that specifically property was not converted right, thus, left the changed line defunc.

This PR changes it back to st until maintainer finishes the refactor

Code examples

from rethinkdb import r
filtering_query_b = (r.row["something"] == "someone") & (r.row["else"] == "else")
print(str(filtering_query_b))

will yield an error of

Unable to serialize object: 'And' object has no attribute 'statement_infix'

Checklist

• I have read and agreed to the RethinkDB Contributor License Agreement

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[lsabi]

Thanks for the contribution!