Add HttpEntity.Head for responding to HEAD requests

[gmethvin]

Fixes #6126

• Adds a new entity type for handling HEAD requests. This allows a user to send a response without a body but with an explicitly defined content type.
• Adds warnings for common error cases when trying to respond to HEAD requests explicitly.

I'm not positive this is the right solution to the problem, though. We need to make sure any solution we provide also doesn't break HEAD requests to existing GET actions, though.

[wsargent]

@gmethvin think we should probably close this