Pull request 125: Improve conditionals

Squashed commit of the following:

commit 0d66b13
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Nov 5 14:28:12 2024 +0300

    httphdr: add etag, fix range

httphdr/httphdr.go

@@ -41,10 +41,10 @@ const (
 //
 // See https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers#conditionals.
 const (
+	ETag              = "ETag"
 	IfMatch           = "If-Match"
 	IfModifiedSince   = "If-Modified-Since"
 	IfNoneMatch       = "If-None-Match"
-	IfRange           = "If-Range"
 	IfUnmodifiedSince = "If-Unmodified-Since"
 	LastModified      = "Last-Modified"
 	Vary              = "Vary"
@@ -97,6 +97,13 @@ const (
 	Forwarded = "Forwarded"
 )
 
+// Common standard headers for range requests.
+//
+// See https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers#range_requests.
+const (
+	IfRange = "If-Range"
+)
+
 // Common standard headers for redirects.
 //
 // See https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers#redirects