typescript-nestjs / Ensure default headers remain unmodified

Created by: luaks

When sending a request with a client generated by typescript-nestjs, the default headers are modified. This occurs when headers such as "Accept" are appended by the client. The root of the issue is that a reference to the default headers is stored instead of a clone.

Previously the code would have been generated like this:

    public deletePet(petId: number, apiKey?: string, ): Observable<any> {
        /* ... */
        let headers = this.defaultHeaders;
        if (apiKey !== undefined && apiKey !== null) {
            headers['api_key'] = String(apiKey);
        }

This adds the api_key to the default headers, which then leaks to all following requests, until another request might change the api_key header.

This PR changes the generated code to:

    public deletePet(petId: number, apiKey?: string, ): Observable<any> {
        /* ... */
        let headers = {...this.defaultHeaders};
        if (apiKey !== undefined && apiKey !== null) {
            headers['api_key'] = String(apiKey);
        }

Instead of changing the default headers, the headers are copied and the header is only set in the scope of the single request.

Mentioning technical comittee for typescript: @TiFu @taxpon @sebastianhaas @kenisteward @Vrolijkx @macjohnny @topce @akehir @petejohansonxo @amakhrov @davidgamero @mkusaka

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  ./mvnw clean package 
  ./bin/generate-samples.sh
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