Fix(lintr): make user-defined lintr-linters can be correctly passed to the R process.

[milanglacier]

if user directly calls

(setq flycheck-lintr-linters
      "linters = with_defaults(line_length_linter=line_length_linter(200),commented_code_linter = NULL)")

with something like this in their config

in deed this will have no effect.

Explicitly specifying"linters=" flycheck-lintr-linters makes things changed

[bbatsov]

I'm fine with the proposed change, but it needs a changelog entry.

[bbatsov]

@milanglacier Friendly ping :-)