Fix warnings from ruby 2.4.2

[dcoxall]

This prevents warnings when accessing initialized instance variables and a warning when redefining the base_url method (attr_accessor defined a reader and then we later define another)

[utilum]

1. This is actually related to when you turn on warnings, not to Ruby 2.4.2. Warnings are on by by default when you launch a rake task since Rake 11.0.0. However, even with Rake 12, the current test suite passes without warnings about instance variable not initialized... May want to add tests for your chnages.