Update ngx-restangular-helper.ts
Update ngx-restangular-helper.ts

Hello, 

To download a file, the responseType is being ignored. Thererfore the responded data is handled as a String, with the normal JSON parsing. In a response with blob data I get an error because it is trying to parse the data as JSON. In adding the responseType
in the createRequestOptions method of RestangularHelper :
```
let requestOptions = new RequestOptions({
      method: RequestMethod[methodName],
      headers: requestHeaders,
      search: requestQueryParams,
      url: options.url,
      body: options.data,
      responseType: options.responseType,
      withCredentials
    });
```
the bug is fixed. We just need to set the reponseType of a request like this :
```
this.restangular.oneUrl('file/').withHttpConfig({responseType: ResponseContentType.Blob}).get( ) .subscribe(data => { this.downloadFileInternal(data) });
```
2muchcoffeecom/ngx-restangular,2muchcoffeecom/ngx-restangular