Allow openai client passthrough (default) behaviour

When the `response_model` parameter in the `chat` method is not defined, it should let the openai client operate as originally intended.

lib/instructor/openai/patch.rb

@@ -29,6 +29,8 @@ def with_retries(max_retries, exceptions, &block)
       # @param validation_context [Hash] The validation context for the parameters. Optional.
       # @return [Object] The processed response.
       def chat(parameters:, response_model: nil, max_retries: 0, validation_context: nil)
+        return json_post(path: '/chat/completions', parameters:) if response_model.nil?
+
         with_retries(max_retries, [JSON::ParserError, Instructor::ValidationError, Faraday::ParsingError]) do
           model = determine_model(response_model)
           function = build_function(model)
@@ -103,7 +105,7 @@ def process_single_response(parsed_response, model)
       # Determines the response model based on the provided value.
       #
       # @param response_model [Class] The response model class or typed array.
-      # @return [Class] The determined response model class.
+      # @return [Class] The response model.
       def determine_model(response_model)
         if response_model.is_a?(T::Types::TypedArray)
           @iterable = true

spec/openai/patch_spec.rb

@@ -188,4 +188,18 @@ def self.name
       end.to raise_error(Instructor::ValidationError)
     end
   end
+
+  describe 'when the client is used ia a standard manner' do
+    it 'does not raise an error when the client is used in a standard manner', vcr: 'patching_spec/standard_usage' do
+      response = patched_client.new.chat(
+        parameters: {
+          model: 'gpt-3.5-turbo',
+          messages: [{ role: 'user', content: 'How is the weather today in New York?' }]
+        }
+      )
+
+      expect(response).to be_a(Hash)
+      expect(response.dig('choices', 0, 'message', 'content')).to be_a(String)
+    end
+  end
 end