Fix RUBY-1051: fails if exists a toplevel Result class
Fix RUBY-1051: fails if exists a toplevel Result class

By using const_defined? instead of defined, we can restrict the searching for Result class under current namespace.

estolfo/mongo-ruby-driver,wandenberg/mongo-ruby-driver,saghm/mongo-ruby-driver,jonhyman/mongo-ruby-driver,jonhyman/mongo-ruby-driver,saghm/mongo-ruby-driver,kay-kim/mongo-ruby-driver,mongodb/mongo-ruby-driver,brandonblack/mongo-ruby-driver,aherlihy/mongo-ruby-driver,kay-kim/mongo-ruby-driver,mongodb/mongo-ruby-driver,estolfo/mongo-ruby-driver,mongodb/mongo-ruby-driver