bson_append_long promises to write an 64 bit number in the header (with the bson_append_estart call), but it writes an 32-bit value.

This was introduced in http://github.com/mongodb/mongo-c-driver/commit/e6eb5ba2cf1d79f0b519834ec5df25c4cd2168c2

To fix, just replace bson_append32 with bson_append64.