Fix Range#cover? returning true for beginless ranges of different
	 types

	Previously `(2..).cover?("2"..)` was false, but
	`(..2).cover?(.."2")` was true.  This changes it so both are false,
	treating beginless ranges the same as endless ranges in regards to
	type checks.

	This also adds documentation to #cover? to describe behavior with
	beginless and endless ranges, testing each documentation example,
	which is how this bug was found.

	Fixes [Bug #18155]
	---
	 range.c                 | 54 ++++++++++++++++++++++++++++++++++++++++++++++++-
	 test/ruby/test_range.rb | 29 ++++++++++++++++++++++++++
	 2 files changed, 82 insertions(+), 1 deletion(-)