Don't force default content-type if no content

[jherven]

Don't force default content-type if no content If response has no content, don't automatically add content-type header

PR Description

• If response has content nothing changes
• If response has no content don't automatically add any content-type header

Addresses #449

[jherven]

@dotnet-policy-service agree

[jherven]

There are some tests that are probably failing here

How could I miss those. Updated

[twsouthwick]

Thanks @jherven for the fix!