Fix handling empty HTTP headers in parser_v1.hpp

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".

test/websocket/stream.cpp

@@ -263,7 +263,7 @@ public:
                     }
                     catch(system_error const& se)
                     {
-                        BEAST_EXPECT(se.code() == ev);
+                        BEAST_EXPECTS(se.code() == ev, se.what());
                     }
                 }
             };