fails:Hash#key? returns true if argument is a key
fails:Hash#key? returns true if the key's matching value was nil
fails:Hash#key? returns true if the key's matching value was false
fails:Hash#key? returns true if the key is nil
fails:Hash#key? compares keys with the same #hash value via #eql?
fails:Hash#key returns the corresponding key for value
fails:Hash#key returns nil if the value is not found
fails:Hash#key doesn't return default value if the value is not found
fails:Hash#key compares values using ==
