ESConfig.toString() output based on server + port, not esUrl

[milesgould]

The toString method in ESConfig reads the deprecated server and port properties. These are overridden by the esUrl property, leading to the following problems:

• if you set esUrl but not server/port, then toString will incorrectly give you "port=9200, server=localhost" (the defaults for server and port)
• if you set server and port but not esUrl, then toString will tell you "port=9203, server=my.awesome.server", but your ES client will actually connect to http://localhost:9200 (the default for esUrl).