Admin API auth and heal related fixes (#5445)

- Fetch region for auth from global state
- Fix SHA256 handling for empty body in heal API

pkg/madmin/heal-commands.go

@@ -138,6 +138,7 @@ func (adm *AdminClient) Heal(bucket, prefix string, healOpts HealOpts,
 	var contentBody io.Reader
 	if clientToken != "" {
 		queryVals.Set("clientToken", clientToken)
+		body = []byte{}
 	} else {
 		// Set a body only if clientToken is not given
 		contentBody = bytes.NewReader(body)