Replace ReadAsStringAsync with ReadAsStreamAsync

[Ggeorgi98]

The size of the HTTP responses can differ and we can get very huge responses. If we store them as a string, they can end up in the Large object heap which can lead to some performance issues most related to the memory.

cloudconvert-dotnet/CloudConvert.API/RestHelper.cs

Lines 27 to 29 in 0edc588

	var response = await _httpClient.SendAsync(request);
	return await response.Content.ReadAsStringAsync();

For reference:

• 'You're (probably still) using HttpClient wrong and it is destabilizing your software', Josef Ottosson - link - you can check the section with the benchmarks and see that ReadAsStreamAsync has performed better than ReadAsStringAsync.