Fix the handling of IntOrString. (#233)

IntOrString has been considered to be 'object' in typescript-fetch,
but it should be specially handled. This allows a similar handling
done in typescript.xml.

openapi/typescript-fetch.xml

@@ -32,6 +32,7 @@
                                 <npmVersion>${generator.client.version}</npmVersion>
                                 <modelPropertyNaming>original</modelPropertyNaming>
                             </configOptions>
+                            <typeMappings>int-or-string=IntOrString</typeMappings>
                         </configuration>
                     </execution>
                 </executions>