Qualify buffer_copy, don't qualify buffer_size:

fix #1416

* Calls to buffer_copy are qualified as net::buffer_copy
* Calls to buffer_size are made unqualified, permitting
  argument dependent lookup to take effect.

test/doc/http_snippets.cpp

@@ -272,10 +272,11 @@ void fxx() {
     auto const cb3 = get_next_chunk_body();
 
     // Manually emit a chunk by first writing the chunk-size header with the correct size
+    using net::buffer_size;
     net::write(sock, chunk_header{
-        net::buffer_size(cb1) +
-        net::buffer_size(cb2) +
-        net::buffer_size(cb3)});
+        buffer_size(cb1) +
+        buffer_size(cb2) +
+        buffer_size(cb3)});
 
     // And then output the chunk body in three pieces ("chunk the chunk")
     net::write(sock, cb1);
@@ -367,7 +368,8 @@ print_cxx14(message<isRequest, Body, Fields> const& m)
             {
                 ec = {};
                 std::cout << buffers(buffer);
-                sr.consume(net::buffer_size(buffer));
+                using net::buffer_size;
+                sr.consume(buffer_size(buffer));
             });
     }
     while(! ec && ! sr.is_done());
@@ -394,7 +396,8 @@ struct lambda
     {
         ec = {};
         std::cout << buffers(buffer);
-        sr.consume(net::buffer_size(buffer));
+        using net::buffer_size;
+        sr.consume(buffer_size(buffer));
     }
 };
 
@@ -435,7 +438,8 @@ split_print_cxx14(message<isRequest, Body, Fields> const& m)
             {
                 ec = {};
                 std::cout << buffers(buffer);
-                sr.consume(net::buffer_size(buffer));
+                using net::buffer_size;
+                sr.consume(buffer_size(buffer));
             });
     }
     while(! sr.is_header_done());
@@ -449,7 +453,8 @@ split_print_cxx14(message<isRequest, Body, Fields> const& m)
                 {
                     ec = {};
                     std::cout << buffers(buffer);
-                    sr.consume(net::buffer_size(buffer));
+                    using net::buffer_size;
+                    sr.consume(buffer_size(buffer));
                 });
         }
         while(! ec && ! sr.is_done());