Make route.Param panic on something else than a string (#183)

As discussed in #181, the values in the context should always be
strings, but this makes sure of it, while the previous implementation
would return an empty string if somehow a non-string value made it
into the context.

Signed-off-by: Bjoern Rabenstein <[email protected]>

Author: beorn7

route/route.go

@@ -25,8 +25,8 @@ type param string
 // Param returns param p for the context, or the empty string when
 // param does not exist in context.
 func Param(ctx context.Context, p string) string {
-	if v, ok := ctx.Value(param(p)).(string); ok {
-		return v
+	if v := ctx.Value(param(p)); v != nil {
+		return v.(string)
 	}
 	return ""
 }