Use correct ModelName default

`openAIChat.generate` needs to use the given `openAIChat`'s model as the default, not a static `gpt-4o-mini"`.

Fixes #80.

Author: ccreutzi

openAIChat.m

@@ -209,7 +209,7 @@
             arguments
                 this                    (1,1) openAIChat
                 messages                      {mustBeValidMsgs}
-                nvp.ModelName           (1,1) string {mustBeModel} = "gpt-4o-mini"
+                nvp.ModelName           (1,1) string {mustBeModel} = this.ModelName
                 nvp.Temperature               {llms.utils.mustBeValidTemperature} = this.Temperature
                 nvp.TopP                      {llms.utils.mustBeValidProbability} = this.TopP
                 nvp.StopSequences             {llms.utils.mustBeValidStop} = this.StopSequences