Address C4927 MSVC warnings for array -> document conversions

[eramongodb]

Followup to #1470. Per MSVC:

scoped_bson.hh(46): warning C4927: illegal conversion; more than one user-defined conversion has been implicitly applied
scoped_bson.hh(46): note: while calling the constructor 'mongocxx::scoped_bson_view::scoped_bson_view(bsoncxx::v1::document::view)'

This might be a false-positive. to_v1(v) returns a v1::array::view, which uses its user-defined conversion function to convert to a v1::document::view, the parameter type requested by the scoped_bson_view ctor. I wonder if the use of {}-initialization without a typename is confusing the compiler into thinking there is another layer of implicit conversion occurring here. Nevertheless, the workaround is trivial (simply making the array -> document conversion explicit).