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

Created by: 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.

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