Restore old 1.8.7 check on completion_proc=

`rb-readline`, the default readline implementation that currently comes
with Windows, does not allow `nil` to be passed in here because it
doesn't respond to `call`. See: https://github.com/ConnorAtherton/rb-readline/blob/9fba246073f78831b7c7129c76cc07d8476a8892/lib/readline.rb#L91-L97

Author: deivid-rodriguez

lib/thor/line_editor/readline.rb

@@ -13,7 +13,10 @@ def self.available?
       def readline
         if echo?
           ::Readline.completion_append_character = nil
-          ::Readline.completion_proc = completion_proc
+          # rb-readline does not allow Readline.completion_proc= to receive nil.
+          if complete = completion_proc
+            ::Readline.completion_proc = complete
+          end
           ::Readline.readline(prompt, add_to_history?)
         else
           super

spec/line_editor/readline_spec.rb

@@ -23,14 +23,14 @@
   describe "#readline" do
     it "invokes the readline library" do
       expect(::Readline).to receive(:readline).with("> ", true).and_return("foo")
-      expect(::Readline).to receive(:completion_proc=)
+      expect(::Readline).to_not receive(:completion_proc=)
       editor = Thor::LineEditor::Readline.new("> ", {})
       expect(editor.readline).to eq("foo")
     end
 
     it "supports the add_to_history option" do
       expect(::Readline).to receive(:readline).with("> ", false).and_return("foo")
-      expect(::Readline).to receive(:completion_proc=)
+      expect(::Readline).to_not receive(:completion_proc=)
       editor = Thor::LineEditor::Readline.new("> ", :add_to_history => false)
       expect(editor.readline).to eq("foo")
     end