Fix broken images API for version <1.7

Simple typo, "ID" should be "Id".  This is causing the images response
to not have IDs in it.

Docker-DCO-1.1-Signed-off-by: Darren Shepherd <darren.s.shepherd@gmail.com> (github: ibuildthecloud)

api/api.go

@@ -222,7 +222,7 @@ func getImagesJSON(eng *engine.Engine, version float64, w http.ResponseWriter, r
 				outLegacy := &engine.Env{}
 				outLegacy.Set("Repository", parts[0])
 				outLegacy.Set("Tag", parts[1])
-				outLegacy.Set("ID", out.Get("ID"))
+				outLegacy.Set("Id", out.Get("Id"))
 				outLegacy.SetInt64("Created", out.GetInt64("Created"))
 				outLegacy.SetInt64("Size", out.GetInt64("Size"))
 				outLegacy.SetInt64("VirtualSize", out.GetInt64("VirtualSize"))