Stop overwriting String#starts_with? and String#ends_with? if defined

Let's not define ruby-openid version of `String#starts_with?` and `String#ends_with?` if they are already defined.
Since ActiveSupport simply aliases Ruby's `start_with?` and `end_with?` into these names 
https://github.com/rails/rails/blob/master/activesupport/lib/active_support/core_ext/string/starts_ends_with.rb, loading ruby-openid under a Rails app causes runtime warning `warning: method redefined; discarding old starts_with?`, and possibly cause some unexpected behavior change for the Rails app.
Of course I confirmed that test/test_extras.rb actually passes against the ActiveSupport version.