minor: defaulting extra_opts to an empty hash in URIParser#connection

Super minor.

I noticed this while working on an issue in MongoMapper last week. I was
creating a client instance from a connection string and was annoying to me
that I had pass an empty hash to this method.

Before:

Mongo::URIParser.new(uri).connection({})

After:

Mongo::URIParser.new(uri).connection