To reproduce:

MongoDB Enterprise > db.c.insert({_id: 0, bson: 100});
WriteResult({ "nInserted" : 1 })
MongoDB Enterprise > db.c.aggregate([{
         $project: {
             _id: 0,
             output: {
                 $convert:
                     {to: {type: "binData", subtype: 2}, input: "$bson", format: "base64"}
             }
         }     
}]);
{ "output" : BinData(2,"AAAAAAAAWUA=") }
MongoDB Enterprise >

A BinData subtype of 2 represents a String.  But that's incorrect for this bindata.

We should be checking the `subtype` field in all conversions to BinData such that they match the intended type.

For example, this function for numbers just blindly passes the subtype to the returned BinData: https://github.com/10gen/mongo/blob/master/src/mongo/db/exec/expression/evaluate_math.cpp#L1481

We should audit all conversions functions to bindata and verify that they check BinData subtypes correctly.