Allows FileUtils.mv to move into a directory.

This addresses the first comment on #40.

lib/fakefs/fileutils.rb

@@ -96,7 +96,8 @@ module FakeFS
 
     def mv(src, dest, options={})
       if target = FileSystem.find(src)
-        FileSystem.add(dest, target.entry.clone)
+        d_path = File.directory?(dest) ? File.join(dest, File.basename(src)) : dest
+        FileSystem.add(d_path, target.entry.clone)
         FileSystem.delete(src)
       else
         raise Errno::ENOENT, src

test/fakefs_test.rb

@@ -797,6 +797,14 @@ class FakeFSTest < Test::Unit::TestCase
     assert_equal('bar', File.open('baz') { |f| f.read })
   end
 
+  def test_mv_to_directory
+    File.open('foo', 'w') {|f| f.write 'bar'}
+    FileUtils.mkdir_p 'destdir'
+    FileUtils.mv 'foo', 'destdir'
+    assert_equal('bar', File.open('destdir/foo') {|f| f.read })
+    assert File.directory?('destdir')
+  end
+
   def test_cp_actually_works
     File.open('foo', 'w') {|f| f.write 'bar' }
     FileUtils.cp('foo', 'baz')