perf(internal/httputil): optimize ContentSubtype function

internal/httputil/http.go

@@ -13,17 +13,23 @@ func ContentType(subtype string) string {
 	return baseContentType + "/" + subtype
 }
 
-// ContentSubtype returns the content-subtype for the given content-type.  The
-// given content-type must be a valid content-type that starts with
-// but no content-subtype will be returned.
-// according rfc7231.
-// contentType is assumed to be lowercase already.
+// ContentSubtype extracts and returns the content subtype from a given Content-Type string.
+// The input is expected to be lowercase, following the conventions of RFC 7231.
+// It handles formats like "type/subtype" or "type/subtype; key=value".
+// If the input is not well-formed, an empty string is returned.
 func ContentSubtype(contentType string) string {
-	left := strings.Index(contentType, "/")
+	switch contentType {
+	case "":
+		return ""
+	case "application/json":
+		return "json"
+	}
+
+	left := strings.IndexByte(contentType, '/')
 	if left == -1 {
 		return ""
 	}
-	right := strings.Index(contentType, ";")
+	right := strings.IndexByte(contentType, ';')
 	if right == -1 {
 		right = len(contentType)
 	}