Check if the header exists and replace it rather than add it again

[flownzu]

Should fix the issue #293

[JohnnyCrazy]

Thanks 👍

I wonder if there is a more performant way, maybe we we need to create HttpRequestMessage objects to set the headers per message. However, we can do this later.

[OmiCron07]

I pulled this merge and now I have an exception with ResumePlayback on this line:
if (_client.DefaultRequestHeaders.Contains(headerPair.Key))

System.InvalidOperationException: 'Misused header name. Make sure request headers are used with HttpRequestMessage, response headers with HttpResponseMessage, and content headers with HttpContent objects.'

[flownzu]

Looks like even checking if the default request header collection contains a certain header validates the header name...

The only way to remove the header and not add duplicate values is to actually clear the whole header collection since Remove checks the header name as well.