Add tests for $arrayToObject behaviour when two keys are identical

According to the documentation, when $arrayToObject is used on an array where some keys repeat, the value of the first key is used.

However, as far as I can see, this behaviour is not being tested for in the jstests integration test.

I propose adding the following test to jstests/aggregation/expressions/arrayToObject.js:

    // check that if two keys are equal, the value of the first key is used

    assert.writeOK(coll.insert({_id: 26, expanded: [["same", 1], ["same", 2]]}));

    result = coll.aggregate([{$match: {_id: 26}}, array_to_object_expr]).toArray();

    assert.eq(result, [{_id: 26, collapsed: {"same": 1}}]);