R16A compatibility

Hello,

I've added single quotes around two atoms in mongo.erl because it fails compilation on R16A. It seems that this compiler version is stricter.

From the docs:

2.3  Atom

An atom is a literal, a constant with name. An atom should be enclosed in single quotes (') if it does not begin with a lower-case letter or if it contains other characters than alphanumeric characters, underscore (_), or @.