[typescript-axios] Fixed call of toISOString on a string type when using string type-mapping

[LiamMartens]

@TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @nicokoenig @topce @akehir @petejohansonxo

Created a detailed repro repository here

Basic gist of it is the following, let's assume you have following swagger definition

"parameters": [
    {
        "name": "param",
        "in": "query",
        "required": false,
        "type": "string",
        "format": "date-time"
    }
],

When type mapping date to string this results in the following generated code

example(param?: string, options: any = {}): RequestArgs {
    ....
    if (param !== undefined) {
        localVarQueryParameter['param'] = (param as any).toISOString(); <-- toISOString() on string type
    }
    ...
},

This same fix was applied to the isDate template.

PR checklist

• [x ] Read the contribution guidelines.
• [ x] If contributing template-only or documentation-only changes which will change sample output, build the project before.
• [x ] Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
• [x ] File the PR against the correct branch: master, 4.3.x, 5.0.x. Default: master.
• [x ] Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

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