add vendoring

vendor/github.com/jhillyerd/enmime/internal/coding/idheader.go

@@ -0,0 +1,26 @@
+package coding
+
+import (
+	"net/url"
+	"strings"
+)
+
+// FromIDHeader decodes a Content-ID or Message-ID header value (RFC 2392) into a utf-8 string.
+// Example: "<foo%3fbar+baz>" becomes "foo?bar baz".
+func FromIDHeader(v string) string {
+	if v == "" {
+		return v
+	}
+	v = strings.TrimLeft(v, "<")
+	v = strings.TrimRight(v, ">")
+	if r, err := url.QueryUnescape(v); err == nil {
+		v = r
+	}
+	return v
+}
+
+// ToIDHeader encodes a Content-ID or Message-ID header value (RFC 2392) from a utf-8 string.
+func ToIDHeader(v string) string {
+	v = url.QueryEscape(v)
+	return "<" + strings.Replace(v, "%40", "@", -1) + ">"
+}