Fix sep:"none" and mapsep:"none"

According to the README.md, a value of `"none"` for the `sep` tag (for
slices) and `mapsep` (for maps) is meant to disable the separator that
would allow multiple entries to be added from a single argument.
However, using `"none"` just set the separator to the rune `'n'`.

Fix the parsing of the tag, and also update `SplitEscaped` to not split
with a separator of `-1`.

Add a new test for the new cases.

mapper.go

@@ -636,6 +636,9 @@ func urlMapper() MapperFunc {
 //
 //     SplitEscaped(`hello\,there,bob`, ',') == []string{"hello,there", "bob"}
 func SplitEscaped(s string, sep rune) (out []string) {
+	if sep == -1 {
+		return []string{s}
+	}
 	escaped := false
 	token := ""
 	for _, ch := range s {