[R-package] Fix custom objective detection in print.lgb.Booster()

[StrikerRUS]

As the best of my knowledge, string comparison in R is case-sensitive. I think it's better to use lowercased "none" string value for objective parameter to avoid errors.

LightGBM/R-package/R/lgb.Booster.R

Lines 247 to 250 in 34f9497

	if (!private$set_objective_to_none) {
	self$reset_parameter(params = list(objective = "none"))
	private$set_objective_to_none <- TRUE
	}

LightGBM/R-package/R/lgb.Booster.R

Lines 864 to 866 in 34f9497

	if (obj == "none") {
	obj <- "custom"
	}

[jameslamb]

I support this change and the simplification in print.lgb.Booster(), thanks!

[github-actions]

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