Make fail_request properly finish the response

[zarqman]

Previously fail_request wrote the initial part of the response, but failed to finish it. When tested with Async::HTTP::Client, this caused the client to experience an EOFError.

This PR causes fail_request to now finish the response. It also rescues exceptions that might arise from the attempt to write the response.

Before:

HTTP/1.1 400 Bad Request

After:

HTTP/1.1 400 Bad Request
connection: close
content-length: 0
                  <- required final blank line

Types of Changes

• Bug fix.

Contribution

• [x] I added tests for my changes.
• [x] I tested my changes locally.
• [x] I agree to the Developer's Certificate of Origin 1.1.