Do not load middleware that's not configured in Sinatra

While honeybadger-io#549
has removed middleware loading for Rails unless configured, it has
also removed the code that prevented the middleware from executing
unless configured.

A similar change is also required for Sinatra, because currently
it tries to execute when not configured.

When a Sinatra app does not use i18n gem and has user feedback
disabled, it errs with a "I18n::InvalidLocale - :en is not a valid
locale" exception from ERB, because Honeybadger tries to render
the feedback form.

lib/honeybadger/init/sinatra.rb

@@ -31,8 +31,8 @@ def install_honeybadger
             if config[:'exceptions.enabled']
               # These two must come before the ErrorNotifier, since an error/response
               # passes through middleware from inner to outer (bottom to top)
-              install_honeybadger_middleware(Honeybadger::Rack::UserFeedback)
-              install_honeybadger_middleware(Honeybadger::Rack::UserInformer)
+              install_honeybadger_middleware(Honeybadger::Rack::UserFeedback) if config[:'feedback.enabled']
+              install_honeybadger_middleware(Honeybadger::Rack::UserInformer) if config[:'user_informer.enabled']
               install_honeybadger_middleware(Honeybadger::Rack::ErrorNotifier)
             end
           end