update deserialize template to handle responseFile

Author: matthewnitschke-wk

modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/json_serializable/api/deserialize.mustache

@@ -1,2 +1,7 @@
 final rawData = _response.data;
-_responseData = rawData == null ? null : deserialize<{{{returnType}}}, {{{returnBaseType}}}>(rawData, '{{{returnType}}}', growable: true);
+{{#isResponseFile}}
+_responseData = rawData == null ? null : rawData as {{{returnType}}};
+{{/isResponseFile}}
+{{^isResponseFile}}
+_responseData = rawData == null ? null : deserialize<{{{returnType}}}, {{{returnBaseType}}}>(rawData, '{{{returnType}}}', growable: true);
+{{/isResponseFile}}