raise Errno::EEXIST when Dir.mkdir is called with an existing directory. Closes #21.

lib/fakefs/dir.rb

@@ -86,6 +86,7 @@ module FakeFS
       parent = string.split('/')
       parent.pop
       raise Errno::ENOENT, "No such file or directory - #{string}" unless parent.join == "" || FileSystem.find(parent.join('/'))
+      raise Errno::EEXIST, "File exists - #{string}" if File.exists?(string)
       FileUtils.mkdir_p(string)
     end
 

test/fakefs_test.rb

@@ -1102,6 +1102,14 @@ class FakeFSTest < Test::Unit::TestCase
     end
   end
 
+  def test_mkdir_raises_error_if_already_created
+    Dir.mkdir "foo"
+
+    assert_raises(Errno::EEXIST) do
+      Dir.mkdir "foo"
+    end
+  end
+
   def test_directory_open
     test = ['.', '..', 'file_1', 'file_2', 'file_3', 'file_4', 'file_5' ]