http_async_protocol_handler: use unsigned type for content length

Resolves comparison between signed and unsigned integer expressions

References cpp-netlib#721

Author: anonimal

boost/network/protocol/http/client/connection/async_protocol_handler.hpp

@@ -416,7 +416,7 @@ struct http_async_protocol_handler {
   bool is_chunk_encoding;
   bool is_chunk_end;
   bool is_content_length;
-  long long content_length;
+  long long unsigned content_length;
 };
 
 }  // namespace impl