[typescript-rxjs]: support reponseType blob

Created by: denyo

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Description of the PR

The goal is to generate the correct responseType which is passed to rxjs/ajax. By default this is json as visible here: https://github.com/ReactiveX/rxjs/blob/master/src/internal/observable/dom/AjaxObservable.ts#L167) but it can be overriden.

I am not too sure about all the possible types but I found json, text, blob and arraybuffer to be supported by rxjs/ajax based on https://github.com/Reactive-Extensions/RxJS-DOM/pull/121/files#diff-d3b897234759c07ed79441f35e56b67fR22. Perhaps it's also possible to pass document and ms-stream as described there for completeness: https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/responseType.

The one that I personally need for a project is blob in order to download an excel file.

The relevant lines for this in the api.json are:

"/api/export": {  
    "get": {
        "produces": ["*/*"],
        "responses": {
            "200": {
                "description": "OK",
                "schema": {  
                    "type": "string",
                    "format": "byte" // <–– here
                }
            }
        }
    }
}

However AbstractTypeScriptClientCodegen maps this to a string because of typeMapping.put("ByteArray", "string");. I tried removing this via this.typeMapping.remove("ByteArray"); in TypeScriptRxjsClientCodegen but then I end up with another import being generated for a model ByteArray which does not exist.

Here are the changes on the call:

-    exportUsingGET = (): Observable<object> => {
-        return this.request<object>({
+    exportUsingGET = (): Observable<ByteArray> => {
+        return this.request<ByteArray>({
             path: '/api/export',
             method: 'GET',
             responseType: 'blob' // <–– that's the one I need
         });
     }

In order to solve this I need some input on how to get there. Or asked differently what's the reason for the mapping of ByteArray to string? Or any how to figure out the appropriate responseType?

@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)