Add default Content-Type: application/json header for custom detector verification request (#4947)

* added content-type=application/json as default header

* improv: set content type header when it is not present in the config

* handled canonicalization

pkg/custom_detectors/custom_detectors.go

@@ -293,6 +293,9 @@ func (c *CustomRegexWebhook) createResults(ctx context.Context, match map[string
 			}
 			req.Header.Add(key, strings.TrimLeft(value, "\t\n\v\f\r "))
 		}
+		if req.Header.Get("Content-Type") == "" {
+			req.Header.Set("Content-Type", "application/json")
+		}
 		resp, err := httpClient.Do(req)
 		if err != nil {
 			continue