Auto detect formatter when client did not specify it

[asok]

Motivation

I was playing with ruby-lsp and Emacs client eglot and I noticed that formatting of the buffer is not working. It seems that the client is not passing the "auto" formatter in the initializationOptions.

I'm very green to lsp so I don't quite understand everything. But when following this thread I've read that the server should apply defaults by itself:

a server should work with an empty literal as well and simply assume a set of defaults

Implementation

Assume by default that the formatter should be auto detected if the initialization option is missing.

Automated Tests

No automated tests yet since I'm opening a draft pull request in order to find out if it's the right way to proceed.

Manual Tests

• Install Emacs 26.3+
• create ~/.emacs.d/init.el file with contents:

  (require 'package)
  (add-to-list 'package-archives '("melpa" . "https://melpa.org/packages/") t)
  (add-to-list 'package-archives '("gnu-devel" . "https://elpa.gnu.org/devel/") t)
  (package-initialize))

  (package-install "eglot")
  (require 'eglot)
  (add-to-list 'eglot-server-programs '((ruby-mode ruby-ts-mode) "ruby-lsp"))

• open a ruby file in Emacs
• run eglot command in Emacs
• run eglot-format-buffer command in Emacs, you should see the error message "Unknown formatter: " in the minibuffer
• apply the patch to your ruby-lsp gem
• run eglot-reconnect command in Emacs
• run eglot-format-buffer command in Emacs, you should not see the error

[vinistock]

Agreed, the server should always set it's defaults internally if nothing is received.

Can you please sign the CLA?

[vinistock]

Is there any scenario where formatter would be an empty string instead of nil?

I don't believe there would be. If it can only be nil, let's move this up instead and do

formatter = options.dig(:initializationOptions, :formatter) || "auto"

[asok]

I have signed the CLA!

[andyw8]

@asok there's a minor linting issue to address.

[vinistock]

Thanks for the contribution! CI is failing, but it's unrelated to these changes.