Fix readline specs

An error was introduced in commit 7061b06
where readline specs fail because `require "readline"` is be called after the
spec has established a mock `::Readline`. Prior to that change
`require "readline"` was always only called once.

This patch:
  * memoizes the `require` to ensure that it only happens once
  * leaves the `Object.const_defined?` check in place to keep tests simple
  * eagerly calls `available?` in the readline spec to match the original
    author's intent by requiring `readline` before the specs start.

Author: bpo

lib/thor/line_editor/readline.rb

@@ -2,8 +2,8 @@ class Thor
   module LineEditor
     class Readline < Basic
       def self.available?
-        begin
-          require "readline"
+        @readline_reqd ||= begin
+            require "readline"
         rescue LoadError
         end
 

spec/line_editor/readline_spec.rb

@@ -2,6 +2,8 @@
 
 describe Thor::LineEditor::Readline do
   before do
+    # Eagerly check Readline availability and cache for this spec
+    Thor::LineEditor::Readline.available?
     unless defined? ::Readline
       ::Readline = double("Readline")
       allow(::Readline).to receive(:completion_append_character=).with(nil)