Better caldav support (#73)

vendor/github.com/samedi/caldav-go/handlers/preconditions.go

@@ -0,0 +1,23 @@
+package handlers
+
+import (
+	"net/http"
+)
+
+type requestPreconditions struct {
+	request *http.Request
+}
+
+func (p *requestPreconditions) IfMatch(etag string) bool {
+	etagMatch := p.request.Header["If-Match"]
+	return len(etagMatch) == 0 || etagMatch[0] == "*" || etagMatch[0] == etag
+}
+
+func (p *requestPreconditions) IfMatchPresent() bool {
+	return len(p.request.Header["If-Match"]) != 0
+}
+
+func (p *requestPreconditions) IfNoneMatch(value string) bool {
+	valueMatch := p.request.Header["If-None-Match"]
+	return len(valueMatch) == 1 && valueMatch[0] == value
+}