Make dumper a bit more verbose

So it displays differences behind interface that would otherwise go
unnoticed (like string vs []byte).

test/diff.go

@@ -5,15 +5,15 @@ import (
 	"github.com/pmezard/go-difflib/difflib"
 )
 
-func init() {
-	spew.Config.SortKeys = true // :\
-}
-
 // Diff diffs two arbitrary data structures, giving human-readable output.
 func Diff(want, have interface{}) string {
+	config := spew.NewDefaultConfig()
+	config.ContinueOnMethod = true
+	config.SortKeys = true
+	config.SpewKeys = true
 	text, _ := difflib.GetUnifiedDiffString(difflib.UnifiedDiff{
-		A:        difflib.SplitLines(spew.Sdump(want)),
-		B:        difflib.SplitLines(spew.Sdump(have)),
+		A:        difflib.SplitLines(config.Sdump(want)),
+		B:        difflib.SplitLines(config.Sdump(have)),
 		FromFile: "want",
 		ToFile:   "have",
 		Context:  3,