refactor: incorporate custom headers into default header setup

Modify the default headers construction to merge in custom headers if provided. This change ensures that custom headers, such as Cloudflare access tokens, are included in the request before self.get_token() is called and without overwriting the default headers like 'Authorization' and 'Content-Type'.

src/keycloak/openid_connection.py

@@ -114,19 +114,18 @@ class KeycloakOpenIDConnection(ConnectionManager):
         self.client_secret_key = client_secret_key
         self.user_realm_name = user_realm_name
         self.timeout = timeout
+        self.headers = {}
+        self.custom_headers = custom_headers
 
         if self.token is None:
             self.get_token()
 
-        self.headers = (
-            {
+        if self.token is not None:
+            self.headers = {
+                **self.headers,
                 "Authorization": "Bearer " + self.token.get("access_token"),
-                "Content-Type": "application/json",
+                "Content-Type": "application/json"
             }
-            if self.token is not None
-            else {}
-        )
-        self.custom_headers = custom_headers
 
         super().__init__(
             base_url=self.server_url, headers=self.headers, timeout=60, verify=self.verify