graphite_default_data is not used for server names consiting of non-alfanumeric characters only

[JoeMcNuggets]

Example config:

http {
   graphite_config ...;
   graphite_default_data nginx.$location;

   server {
     server_name _; # can also be ___ or @@@
     ...

     location / {
        # no graphite_data directive here
        ...
     }
  }
}

Actual behavior:

• module changes all the alphanumeric characters in server to _
• module tries to strip leading and following characters _ and as a result server_name len becomes 0
• module skips this server's locations instead of adding default_data to them

Expected behavior:

• module adds default_data to locations in servers with names like _, ___, @@@