New constructors for message:

The message class now behaves like a pair with respect to the construction
of the body and headers. Additional constructors allow construction of
just the body portion from a tuple, leaving the headers default
constructed.

Previous constructors are removed as they were a notational convenience
for assembling HTTP/1 requests and responses. They are not necessary
as this library aims at library writers and not end users.

examples/http_crawl.cpp

@@ -46,7 +46,10 @@ int main(int, char const*[])
             stream<ip::tcp::socket> hs(ios);
             connect(hs.lowest_layer(), it);
             auto ep = hs.lowest_layer().remote_endpoint();
-            request_v1<empty_body> req({"GET", "/", 11});
+            request_v1<empty_body> req;
+            req.method = "GET";
+            req.url = "/";
+            req.version = 11;
             req.headers.insert("Host", host +
                 std::string(":") + std::to_string(ep.port()));
             req.headers.insert("User-Agent", "beast/http");