Remove msgtype requirement in HttpMessageContent

[jods4]

HttpMessageContent serializes HTTP requests/responses inside a mulitpart/mixed body.

For requests, it sends the header Content-Type: application/http; msgtype=request.
For responses, it expects the header Content-Type: application/http; msgtype=response.

This is where msgtype=request is added in source:

AspNetWebStack/src/System.Net.Http.Formatting/HttpMessageContent.cs

Line 68 in 1231b77

Headers.ContentType.Parameters.Add(new NameValueHeaderValue(MsgTypeParameter, DefaultRequestMsgType));

This is where msgtype=response is validated in source:

AspNetWebStack/src/System.Net.Http.Formatting/HttpMessageContent.cs

Lines 142 to 171 in 1231b77

	foreach (NameValueHeaderValue parameter in contentType.Parameters)
	{
	if (parameter.Name.Equals(MsgTypeParameter, StringComparison.OrdinalIgnoreCase))
	{
	string msgType = FormattingUtilities.UnquoteToken(parameter.Value);
	if (!msgType.Equals(isRequest ? DefaultRequestMsgType : DefaultResponseMsgType, StringComparison.OrdinalIgnoreCase))
	{
	if (throwOnError)
	{
	throw Error.Argument("content", Properties.Resources.HttpMessageInvalidMediaType, FormattingUtilities.HttpContentType.Name, isRequest ? DefaultRequestMediaType : DefaultResponseMediaType);
	}
	else
	{
	return false;
	}
	}
	return true;
	}
	}
	}
	if (throwOnError)
	{
	throw Error.Argument("content", Properties.Resources.HttpMessageInvalidMediaType, FormattingUtilities.HttpContentType.Name, isRequest ? DefaultRequestMediaType : DefaultResponseMediaType);
	}
	else
	{
	return false;
	}

HttpMessageContent should be configurable to not sent a msgtype parameter, and accept responses without a msgtype (given Content-Type: application/http).

As a common real-world example, the OData specification uses Content-Type: application/http without any further msgtype in both its batched requests and responses, see:
https://www.odata.org/documentation/odata-version-2-0/batch-processing/

Inside Microsoft, Dataverse exposes OData services and its batching endpoint is unusable with HttpMessageContent because:

• Dataverse throws when a batch has msgtype=request in its Content-Type;
• ReadAsHttpResponseMessageAsync() throws when parsing Dataverse response because it does not have msgtype=response.

[MackinnonBuck]

Thanks for reaching out. We are currently prioritizing critical bug fixes and security issues over new features in this repository. Putting this in the backlog to collect more feedback.