Use `Net::HTTPClientException` instead of `Net::HTTPServerException`

`Net::HTTPServerException` has been deprecated since r63590. [Bug #14688]
And `net/http/responses.rb` uses the deprecated constant, so Ruby warns of the deprecation.

Example:

```bash
$ ruby -r net/http -e ''
/home/pocke/.rbenv/versions/trunk/lib/ruby/2.6.0/net/http/responses.rb:22: warning: constant Net::HTTPServerException is deprecated
```

This change suppresses the warning.
From: Masataka Pocke Kuwabara <kuwabara@pocke.me>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63591 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/net/http/responses.rb

@@ -19,7 +19,7 @@ class Net::HTTPRedirection < Net::HTTPResponse           # 3xx
 end
 class Net::HTTPClientError < Net::HTTPResponse           # 4xx
   HAS_BODY = true
-  EXCEPTION_TYPE = Net::HTTPServerException   # for backward compatibility
+  EXCEPTION_TYPE = Net::HTTPClientException   # for backward compatibility
 end
 class Net::HTTPServerError < Net::HTTPResponse           # 5xx
   HAS_BODY = true