Ensure that SSGI response bodies are renderable

gistfile1.scala

scala> import org.scalatra.ssgi._      
import org.scalatra.ssgi._

scala> val res = Response(body = "foo")
res: org.scalatra.ssgi.Response[java.lang.String] = Response(200,Map(),foo)

scala> res map { _.toUpperCase }       
res10: org.scalatra.ssgi.Response[java.lang.String] = Response(200,Map(),FOO)

scala> res map { x => <h1>{x}</h1> }   
res11: org.scalatra.ssgi.Response[scala.xml.Elem] = Response(200,Map(),<h1>foo</h1>)

scala> class Wtf
defined class Wtf

scala> res map { x => new Wtf }                                 
<console>:20: error: could not find implicit value for evidence parameter of type (Wtf) => org.scalatra.ssgi.Renderable
       res map { x => new Wtf }
           ^

scala> implicit def wtf2Renderable(wtf: Wtf) = new Renderable {}
wtf2Renderable: (wtf: Wtf)java.lang.Object with org.scalatra.ssgi.Renderable

scala> res map { x => new Wtf }                                 
res14: org.scalatra.ssgi.Response[Wtf] = Response(200,Map(),Wtf@1c093b3c)