Range types Set method supports its own type, string, and nil

Previously Set would always return an error when called on a range type.
Now it will accept an instance of itself, a pointer to an instance of
itself, a string, or nil. Strings are parsed with the same logic as
DecodeText.

numrange.go

@@ -16,7 +16,24 @@ type Numrange struct {
 }
 
 func (dst *Numrange) Set(src interface{}) error {
-	return errors.Errorf("cannot convert %v to Numrange", src)
+	// untyped nil and typed nil interfaces are different
+	if src == nil {
+		*dst = Numrange{Status: Null}
+		return nil
+	}
+
+	switch value := src.(type) {
+	case Numrange:
+		*dst = value
+	case *Numrange:
+		*dst = *value
+	case string:
+		return dst.DecodeText(nil, []byte(value))
+	default:
+		return errors.Errorf("cannot convert %v to Numrange", src)
+	}
+
+	return nil
 }
 
 func (dst Numrange) Get() interface{} {