Dedicated type for conditional match header fields

The `If-Match` and `If-None-Match` conditional headers can have either a
wildcard or a (quoted) ETag as value. However, the ETag _could_ be a
literal `*`, so care must be taken to allow these cases to be
distinguished. The values of these headers have to be handled by the
backend, so export a type that facilitates working with these values.

elements.go

@@ -30,3 +30,24 @@ type groupMembership struct {
 	XMLName xml.Name        `xml:"DAV: group-membership"`
 	Hrefs   []internal.Href `xml:"href"`
 }
+
+// ConditionalMatch represents the value of a conditional header
+// according to RFC 2068 section 14.25 and RFC 2068 section 14.26
+// The (optional) value can either be a wildcard or an ETag.
+type ConditionalMatch string
+
+func (val ConditionalMatch) IsSet() bool {
+	return val != ""
+}
+
+func (val ConditionalMatch) IsWildcard() bool {
+	return val == "*"
+}
+
+func (val ConditionalMatch) ETag() (string, error) {
+	var e internal.ETag
+	if err := e.UnmarshalText([]byte(val)); err != nil {
+		return "", err
+	}
+	return string(e), nil
+}