Make the recommended name formal

`HTTPServerException` is the name deprecated since years ago.

Author: nobu

lib/net/http/exceptions.rb

@@ -16,13 +16,13 @@ class Net::HTTPError < Net::ProtocolError
 class Net::HTTPRetriableError < Net::ProtoRetriableError
   include Net::HTTPExceptions
 end
-class Net::HTTPServerException < Net::ProtoServerError
+class Net::HTTPClientException < Net::ProtoServerError
   # We cannot use the name "HTTPServerError", it is the name of the response.
   include Net::HTTPExceptions
 end
 
 # for compatibility
-Net::HTTPClientException = Net::HTTPServerException
+Net::HTTPServerException = Net::HTTPClientException
 
 class Net::HTTPFatalError < Net::ProtoFatalError
   include Net::HTTPExceptions