Add IElasticClient.TemplateExists string overload

[niemyjski]

I noticed today that I was getting an error when I tried to delete a template

So I went and tried wrapping it in an if block and call TemplateExists(string). However, there isn't an overload that just takes a string. GetTemplate, PutTemplate and DeleteTemplate have a string overload.

[gmarz]

Added. Good catch @niemyjski 👍

[niemyjski]

Thanks :), Do you want me to close this issue?

[gmarz]

No problem. It'll be closed once the PR is merged.