Add <!-- as magic bytes for SVG detection

CHANGELOG.md

@@ -1,6 +1,8 @@
 # Changelog
 
 ## [Unreleased]
+### Fixed
+- Fix detection of SVG starting with a comment.
 
 ## [2.9.0] - 2020-01-30
 ### Added

imagemeta/svg.go

@@ -12,4 +12,6 @@ func init() {
 	}
 	RegisterFormat("<?xml ", decodeMeta)
 	RegisterFormat("<svg", decodeMeta)
+	// We believe that file starting with HTML comment is SVG
+	RegisterFormat("<!--", decodeMeta)
 }