This patch rectifies flush() of beast::http::parser_v1
to properly handle the case when an HTTP header has
empty value.
Without the fix an empty-valued HTTP header is being
concatenated with the header directly following it.
This situation can be replicated using eg. curl:
curl <url> -H "X-First;" -H "X-Second: bla"
What Beast's client would see is a single header named
as "X-FirstX-Second".