Fix race condition in CommandMap#prefix

`PrefixProvider#[]` could return `nil` in multi-threaded scenarios due to a race
condition involving `||=`, which is not an atomic operation in JRuby.

The test that I added failed with JRuby 9.0.4.0 before the fix; now it passes.

lib/sshkit/command_map.rb

@@ -28,8 +28,6 @@ def initialize
 
       def [](command)
         @storage[command] ||= []
-
-        @storage[command]
       end
     end
 

test/unit/test_command_map.rb

@@ -56,5 +56,14 @@ def test_indifferent_prefix
       assert_equal map[:rake], "/home/vagrant/.rbenv/bin/rbenv exec bundle exec rake"
     end
 
+    def test_prefix_initialization_is_thread_safe
+      map = CommandMap.new
+      threads = Array.new(3) do
+        Thread.new do
+          (1..1_000).each { |i| assert_equal([], map.prefix[i.to_s]) }
+        end
+      end
+      threads.each(&:join)
+    end
   end
 end