don't overwrite existing directories yet

lib/fakefs.rb

@@ -43,7 +43,7 @@ module FakeFS
     end
 
     def self.exists?(path)
-      FileSystem.find(path)
+      FileSystem.find(path) || false
     end
 
     def self.directory?(path)
@@ -152,7 +152,7 @@ module FakeFS
 
       object.name = parts.last
       object.parent = d
-      d[parts.last] = object
+      d[parts.last] ||= object
     end
 
     # copies directories and files from the real filesystem

test/fakefs_test.rb

@@ -36,6 +36,13 @@ class FakeFSTest < Test::Unit::TestCase
     assert File.directory?(path)
   end
 
+  def test_doesnt_overwrite_existing_directories
+    FileUtils.mkdir_p(path = "/path/to/dir")
+    assert File.exists?(path)
+    FileUtils.mkdir_p("/path/to")
+    assert File.exists?(path)
+  end
+
   def test_can_create_symlinks
     FileUtils.mkdir_p(target = "/path/to/target")
     FileUtils.ln_s(target, "/path/to/link")