HTTP: retry failed HEAD requests too

Mark's new document about HTTP Retries
(https://mnot.github.io/I-D/httpbis-retry/) made me check our code and I
spotted that we don't retry failed HEAD requests which seems totally
inconsistent and I can't see any reason for that separate treatment.

So, no separate treatment for HEAD starting now. A HTTP request sent
over a reused connection that gets cut off before a single byte is
received will be retried on a fresh connection.

Made-aware-by: Mark Nottingham