Issue #346 Fixed bug where MkDeployKeys XML nodes were not being written

There were a few things that were incorrect. Fixes done are:

1. Adding the `<deploykeys/>` node if not present (`addIf` call in `MkDeployKeys` constructor).
2. Fixing the XPath strings to look for the `coords` _attribute_ instead of _XML nodes_.
3. Converted XML node names and XPath strings to all-lowercase (`deployKeys` to deploykeys`). 

As for 3. above, Xembly converts them to lowercase when generating directives. This means I can create a node by using `add("deployKeys")` and the resulting XML will be `<deploykeys/>`. However, I can't retrieve the same node when I reference `deployKeys` in XPath (e.g. when you call `xml().nodes("/deployKeys")`. This seems like a bug either in Jcabi-XML or Xembly; I think it's the latter because as far as I know XML node names are case sensitive.

 @yegor256 what do you think about the XML behavior? I can create an issue in either Xembly or Jcabi-XML depending on where you think it's a valid bug.