accept protobuf encoding (#1096)

Co-authored-by: rodolphe <rodolphe@zen.ly>

httplib.h

@@ -2867,6 +2867,7 @@ inline bool can_compress_content_type(const std::string &content_type) {
          content_type == "application/javascript" ||
          content_type == "application/json" ||
          content_type == "application/xml" ||
+         content_type == "application/protobuf" ||
          content_type == "application/xhtml+xml";
 }