Accept user_id as an int
Accept user_id as an int

Since it's natural to provide user ids as ints, we shouldn't choke on
them when we get them.  This patch converts them to a string.

Test Plan:

    1. instantiate a handle, passing in an int as your user ID

        ab = AcademicBenchmarks::Api::Handle.new(partner_id:
        'my-id', partner-key: 'my-key', user_id: 5)

    2. use the handle to make a query:

        ab.standards.authorities

    3. change the user id to another int:

        ab.user_id = 12

    4. Repeat step 2 and see that it works

Change-Id: I913390401c70a08475d03f9b51b838de50075e18

instructure/academic_benchmarks,instructure/academic_benchmarks