Don't set content-type if not content

[jherven]

Summary

Using this library in a dotnet core application forcefully adds content-type "text/html" (when not set) even if there's no content. This isn't always desired and I think it's neither expected when there is no content for the response.

Motivation and goals

• Some clients break when there is a content-type set but no content
• At least for my migrated projects this doesn't mimic the behavior I had in the old application

Risks / unknowns

If somebody already using this library relies on this header always being set this change may break something

Detailed design

The change I've done for now is very simple.

src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/SetDefaultResponseHeadersMiddleware.cs -> WriteDefaultContentType

Added new criteria "hasContent"

bool hasContent = context.Response.ContentLength.HasValue;
        if (hasContent && context.Response.Headers.ContentType.Count == 0)
        {
            context.Response.Headers.ContentType = "text/html";
        }

[twsouthwick]

Thanks @jherven - have you verified this is the same behavior as on ASP.NET Framework? If so, would you be willing to submit a PR?

[jherven]

Yes, for the scenarios I tested this change better mimics ASP.NET framework. PR created #450 .