Fixed string escaping omissions (#599)

I received a `RegexpError` when I typed `::Array[`.
::Array[/Users/mi/ghq/github.com/ruby/irb/lib/irb/completion.rb:236:in `retrieve_completion_data': premature end of char-class: /^Array[/ (RegexpError)

Author: ima1zumi

lib/irb/completion.rb

@@ -233,7 +233,7 @@ def self.retrieve_completion_data(input, bind: IRB.conf[:MAIN_CONTEXT].workspace
         if doc_namespace
           candidates.find { |i| i == receiver }
         else
-          candidates.grep(/^#{receiver}/).collect{|e| "::" + e}
+          candidates.grep(/^#{Regexp.quote(receiver)}/).collect{|e| "::" + e}
         end
 
       when /^([A-Z].*)::([^:.]*)$/

test/irb/test_completion.rb

@@ -310,6 +310,7 @@ def test_complete_absolute_constants_with_special_characters
       assert_empty(IRB::InputCompletor.retrieve_completion_data("::A.", bind: binding))
       assert_empty(IRB::InputCompletor.retrieve_completion_data("::A(", bind: binding))
       assert_empty(IRB::InputCompletor.retrieve_completion_data("::A)", bind: binding))
+      assert_empty(IRB::InputCompletor.retrieve_completion_data("::A[", bind: binding))
     end
 
     def test_complete_reserved_words