Manuel Covas

[typescript-fetch] Restore pre-es2017 compatibility in modelEnum template

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The `instanceOf{{classname}}` function introduced in 93b76dde371b1e3e111040541ef568a329b62be6 makes use of the [Object.values](https://262.ecma-international.org/8.0/#sec-object.values) function introduced in [ECMAScript 2017](https://262.ecma-international.org/8.0), thus breaking compatibility with prior versions such as ES2016 and ES2015. This PR suggests...