lib/pry/commands/ls.rb: fix `ls -c` on top-level

This is rather an odd bug introduced by this commit: "Add support for
BasicObjects to `ls`"[1]. `ls -c` works, if you install it as a gem, but
it refuses to display anything, if you run it via `rake pry`. Moreover,
I've tried to write a failing test without changing the implementation,
but the test suite couldn't fail. Let's fix this issue, because it can
save us a lot of time, avoiding the situation when one stumble upon this
oddity once again.

[1]: ac683892d1

lib/pry/commands/ls.rb

@@ -185,7 +185,20 @@ class Pry
     end
 
     def module_to_interrogate
-      interrogating_a_module? ? object_to_interrogate : (class << object_to_interrogate; self.ancestors.first; end)
+      if interrogating_a_module?
+        object_to_interrogate
+      elsif instance_of_object?
+        object_to_interrogate.class
+      else
+        class << object_to_interrogate; self.ancestors.first; end
+      end
+    end
+
+    def instance_of_object?
+      superclass = object_to_interrogate.instance_eval do
+        (class << self; self; end).superclass
+      end
+      superclass == Object
     end
 
     def write_out_globals

spec/commands/ls_spec.rb

@@ -141,6 +141,13 @@ describe "ls" do
   end
 
   describe "constants" do
+    it "works on top-level" do
+      toplevel_consts = pry_eval('ls -c')
+      [/RUBY_PLATFORM/, /ARGF/, /STDOUT/].each do |const|
+        toplevel_consts.should =~ const
+      end
+    end
+
     it "should show constants defined on the current module" do
       pry_eval("class TempFoo1; BARGHL = 1; end", "ls TempFoo1").should =~ /BARGHL/
     end