allow procs as command map values

so this example works properly:

  SSHKit.config.command_map[:bundle] = -> { "#{fetch(:ruby_cmd)} /usr/bin/local/bundle" }

lib/sshkit/command_map.rb

@@ -33,18 +33,20 @@ def [](command)
       end
     end
 
+    TO_VALUE = ->(obj) { obj.respond_to?(:call) ? obj.call : obj }
+
     def initialize(value = nil)
       @map = CommandHash.new(value || defaults)
     end
 
     def [](command)
       if prefix[command].any?
-        prefixes = prefix[command].map{ |prefix| prefix.respond_to?(:call) ? prefix.call : prefix }
+        prefixes = prefix[command].map(&TO_VALUE)
         prefixes = prefixes.join(" ")
 
         "#{prefixes} #{command}"
       else
-        @map[command]
+        TO_VALUE.(@map[command])
       end
     end
 

test/unit/test_command_map.rb

@@ -16,6 +16,15 @@ def test_setter
       assert_equal map[:rake], "/usr/local/rbenv/shims/rake"
     end
 
+    def test_setter_procs
+      map = CommandMap.new
+      i = 0
+      map[:rake] = -> { i += 1; "/usr/local/rbenv/shims/rake#{i}" }
+
+      assert_equal map[:rake], "/usr/local/rbenv/shims/rake1"
+      assert_equal map[:rake], "/usr/local/rbenv/shims/rake2"
+    end
+
     def test_prefix
       map = CommandMap.new
       map.prefix[:rake].push("/home/vagrant/.rbenv/bin/rbenv exec")