Handle continuation HTTP headers correctly

[acg]

Currently, http_parser.parser joins continued header values together as a comma-separated list. According to the RFC continuation lines should instead be joined with a single space:

Any LWS that occurs between field-content MAY be replaced with a single SP before interpreting the field value or forwarding the message downstream.

If a header name is repeated, the values can be folded together into a comma-separated list. The current code handles this correctly.

But in either case, if the header value so far is empty, I don't think we should prepend the separator. So this:

X-Test:
X-Test: foo
X-Test: bar

Should be equivalent to

X-Test: foo, bar

Not

X-Test: , foo, bar

I've also attached a couple test suite functions for the continuation and repeated header cases.