Resolve g++ compilation error when using standalone asio

asio_compatibility.hpp

@@ -7,7 +7,6 @@
 #include <asio.hpp>
 #include <asio/steady_timer.hpp>
 namespace SimpleWeb {
-  namespace asio = asio;
   using error_code = std::error_code;
   using errc = std::errc;
   using system_error = std::system_error;

tests/io_test.cpp

@@ -3,6 +3,10 @@
 #include "server_http.hpp"
 #include <future>
 
+#ifndef USE_STANDALONE_ASIO
+namespace asio = boost::asio;
+#endif
+
 using namespace std;
 
 using HttpServer = SimpleWeb::Server<SimpleWeb::HTTP>;
@@ -359,7 +363,7 @@ int main() {
         }
         else if(call_num == 3) {
           ASSERT(response->content.string().empty());
-          ASSERT(ec == SimpleWeb::asio::error::eof);
+          ASSERT(ec == asio::error::eof);
         }
         ++call_num;
       });