typescript-fetch: Fix type errors in generated code

Created by: gbark

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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@TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @nicokoenig (2018/09) @topce (2018/10) @akehir (2019/07)

Description of the PR

Fixes an issue where typescript-fetch in 4.1.0 generates Typescript code which doesn't compile.

src/api/petstore/src/runtime.ts:186:9 - error TS2322: Type 'HTTPHeaders | undefined' is not assignable to type 'HTTPHeaders'.
  Type 'undefined' is not assignable to type 'HTTPHeaders'.

186         return this.configuration.headers;
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

src/api/petstore/src/runtime.ts:190:9 - error TS2322: Type '"omit" | "same-origin" | "include" | undefined' is not assignable to type 'RequestCredentials'.
  Type 'undefined' is not assignable to type 'RequestCredentials'.

190         return this.configuration.credentials;
            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The solution is to simply add undefined as a potential return type to headers and credentials properties of the Configurationclass in runtime.ts.

fixes #3604 (closed)