Fix completion deleting the triggerChar

[alexheretic]

Fixes incorrect completion, e.g. .with -> with_foo() (dot deletion).

Looking at the server messages they seem correct, so this does indeed seem like a client bug. Why was the triggerChar.length being removed? Was this a workaround for some language-server?

This fix is also available in my fork:

"dependencies": {
  "atom-languageclient": "github:alexheretic/atom-languageclient#build",
}