Fix some problematic class comparisons

[MichaelChirico]

class() is not guaranteed to return an object of length 1, so statements like if (class(x) == "y") are problematic since this is often a vector comparison inside a scalar logical operator (if).

I fixed a few of these for illustration, but many more remain in the package. You can see them with lintr::lint_package(linters = lintr::class_equals_linter()).