Omit Server header when server_name is empty

According to RFC 9110 Section 10.2.4, the Server header field
in responses is optional (MAY be sent).

This change modifies the connection logic to prevent automatically
adding the 'Server' header if the application-wide server_name
is configured to be an empty string via `app.server_name("")`.

Addresses issue #1020

Author: Sightem

include/crow/app.h

@@ -329,7 +329,7 @@ namespace crow
             return *this;
         }
 
-        /// \brief Set the server name
+        /// \brief Set the server name included in the 'Server' HTTP response header. If set to an empty string, the header will be omitted by default.
         self_t& server_name(std::string server_name)
         {
             server_name_ = server_name;

include/crow/http_connection.h

@@ -365,7 +365,7 @@ namespace crow
                 buffers_.emplace_back(content_length_.data(), content_length_.size());
                 buffers_.emplace_back(crlf.data(), crlf.size());
             }
-            if (!res.headers.count("server"))
+            if (!res.headers.count("server") && !server_name_.empty())
             {
                 static std::string server_tag = "Server: ";
                 buffers_.emplace_back(server_tag.data(), server_tag.size());