delegate to Rack::Request to get IP address

* add IssueRenderers::Rack#request
* test #request, #ip_address
use Rack's request_method, add #url test
delegate to Rack::Request#params

* use Rack::MockRequest to mock query string
* rename local variables to match class
delegate to Rack::Request#session, add #session test
delegate to Rack::Request#user_agent, add #http_headers test
Merge pull request #76 from teeparham/rack

Use Rack Request