Avoid calling >= on unknown receivers in ls command

[dejmedus]

If Ls#dump_methods is called with an obj whose ancestors alter >=, the comparison (c >= Object/c >= Class) may raise an error. For example:

class Foo; class << self; undef >=; end; end
foo = Foo.new
ls foo

Foo >= Object # NoMethodError     

To prevent this, we could defensively flip the comparison so that <= is called on known receivers

[tompng]

To solve the problem, the change looks good. However, ls foo ls Foo doesn't work when some other methods are missing:

<
constants
class_variables
to_s
ancestors
public_instance_methods

Unlike Object instances that we need to consider BasicObject, I think we can assume Classes doesn't undef/change most of these methods: constants, class_variables, to_s, ancestors, public_instance_methods.
But for <, I guess it might be removed and need to change if klass < Object to if Object > klass in the situation you are assuming.

Can you share what kind of situation :<= is removed? Will :> be removed too?
Do you know an existing gem that creates a class without :<=?
The correct fix depends on the situation what you are assuming, and needs to be commented in test code.