Fixed set_data issue.(duped notification when using notify_change)

# steps
1. Run this code.
```ruby:test.rb
require 'win32/clipboard'

@count = 0
Thread.start do
  Win32::Clipboard.notify_change{@count += 1; puts "count:#{@count}"}
end

sleep(1)
Win32::Clipboard.set_data('foo')
sleep(1)
```

Results:
```
count:1
count:2
```

# problem
It should be only "count:1",
but "count:1" and "count:2".

I'm using the following:

Windows 7 (x64)
Ruby 2.0.0p353 (2013-11-22)[x64-mingw32]
win32/clipboard 0.6.2