FIXED headers are built and returned properly

invoice_ninja/__init__.py

@@ -29,7 +29,7 @@ class InvoiceNinja(object):
                  api_token: str = str()):
         self.endpoint_url = '{}/{}'.format(endpoint_url, self.API_V1)
         self.api_token = api_token
-        self.headers = dict()
+        self.headers = self.build_headers()
 
     def _get_url_for(self, endpoint: str = 'ping'):
         '''
@@ -57,14 +57,14 @@ class InvoiceNinja(object):
                 'X-API-TOKEN': self.api_token,
                 'X-Requested-With': 'XMLHttpRequest'}
 
-        return self.headers.update(headers)
+        return headers
 
     def ping(self):
         '''
             Ping Invoice Ninja instance.
         '''
         server_response = requests.get(url=self._get_url_for(),
-                headers=self.build_headers())
+                headers=self.headers)
 
         if server_response.ok:
             return True