[dart-dio][client] Handle x-www-form-urlencoded form data (OpenAPITools#5597)

Author: adg-mh

modules/openapi-generator/src/main/resources/dart-dio/api.mustache

@@ -55,6 +55,9 @@ class {{classname}} {
                 }
             {{/isFile}}
         {{/isMultipart}}
+        {{^isMultipart}}
+            formData['{{baseName}}'] = parameterToString(_serializers, {{paramName}});
+        {{/isMultipart}}
         {{/formParams}}
         bodyData = FormData.fromMap(formData);
         {{/hasFormParams}}

samples/client/petstore/dart-dio/.openapi-generator/VERSION

@@ -1 +1 @@
-4.3.1-SNAPSHOT
+4.3.1-SNAPSHOT

samples/client/petstore/dart-dio/lib/api/pet_api.dart

@@ -266,6 +266,8 @@ class PetApi {
         List<String> contentTypes = ["application/x-www-form-urlencoded"];
 
         Map<String, dynamic> formData = {};
+            formData['name'] = parameterToString(_serializers, name);
+            formData['status'] = parameterToString(_serializers, status);
         bodyData = FormData.fromMap(formData);