[dart][dart-dio] Fix missing byte response cast (#8985)

There doesn't seem to be an example.

Author: kuhnroyal

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

@@ -108,7 +108,7 @@ class {{classname}} {
     {{#returnType}}
 
     {{#isResponseFile}}
-    final {{{returnType}}} _responseData = _response.data;
+    final {{{returnType}}} _responseData = _response.data as {{{returnType}}};
     {{/isResponseFile}}
     {{^isResponseFile}}
     {{#returnSimpleType}}