Quell ruby 2.1.0-preview2's deprecation warning about File.exists?

It says: "warning: File.exists? is a deprecated name, use File.exist? instead"
Update tests to expect the Net::HTTP behavior in forthcoming versions of JRuby

The latest build identifies itself as Ruby 2.1.0, although its Net::HTTP
implementation still diverges a bit from MRI (due to having a different
underlying SSL implementation, I assume).