Fix FileUtils.cp behavior.

lib/fakefs.rb

@@ -29,10 +29,6 @@ def cp(src, dest)
       dst_file = FileSystem.find(dest)
       src_file = FileSystem.find(src)
 
-      if dst_file
-        raise Errno::EEXIST, dest
-      end
-
       if !src_file
         raise Errno::ENOENT, src
       end
@@ -41,7 +37,12 @@ def cp(src, dest)
         raise Errno::EISDIR, src
       end
 
-      FileSystem.add(dest, src_file.entry.clone)
+      if dst_file and File.directory?(dst_file)
+        FileSystem.add(File.join(dest, src), src_file.entry.clone)
+      else
+        FileSystem.delete(dest)
+        FileSystem.add(dest, src_file.entry.clone)
+      end
     end
 
     def cp_r(src, dest)

test/fakefs_test.rb

@@ -304,13 +304,20 @@ def test_cp_actually_works
     assert_equal 'bar', File.read('baz')
   end
 
-  def test_cp_fails_on_dest_exists
+  def test_cp_file_into_dir
     File.open('foo', 'w') {|f| f.write 'bar' }
     FileUtils.mkdir_p 'baz'
 
-    assert_raise Errno::EEXIST do
-      FileUtils.cp('foo', 'baz')
-    end
+    FileUtils.cp('foo', 'baz')
+    assert_equal 'bar', File.read('baz/foo')
+  end
+
+  def test_cp_overwrites_dest_file
+    File.open('foo', 'w') {|f| f.write 'FOO' }
+    File.open('bar', 'w') {|f| f.write 'BAR' }
+
+    FileUtils.cp('foo', 'bar')
+    assert_equal 'FOO', File.read('bar')
   end
 
   def test_cp_fails_on_no_source