We won't add operator[] to StringRef.  This is because it may be
undefined if pos == size(), and StringRef's base + len does not point
to the valid region.  This solely depends on the given buffer, so we
cannot do anything to fix.  For workaround, if we need this kind of
operator, we may add it under another name, like char_at(size_type).

RFC 7540 says that proxy should not emit :authority when translating
HTTP/1 request in origin or asterisk form to HTTP/2.  To keep this
semantics in tact, we should also refrain from emitting :authority if
it is missing (host header field is required in this case).

For HTTP/2, we do this validation in libnghttp2.  http-parser does
this partially, when it parses URI, but it does not do anything for
Host header field.  libspdylay does not perform anything.  So do some
additional validation for HTTP/1 and SPDY cases.  integration tests
were also added to make sure they work.

Without this validation, test spuriously succeeds, but in fact it
doesn't.  This might occur if stream is closed with RST_STREAM or
GOAWAY.

We only change req_.upgrade_request once, so just using else is
suffice.