Kernel open hijacking

It seems that FakeFS cannot handle open-uri on files.

```ruby
require 'open-uri'
require 'fakefs/safe'

describe 'fakefs and open-uri' do
  before(:each) { FakeFS.activate! }
  after(:each) { FakeFS.deactivate! }

  it 'can read files' do
    FileUtils.mkdir_p '/tmp'
    File.open('/tmp/a.b', 'w+') { |f| f.puts 'test' }

    data = File.read('/tmp/a.b')
    expect(data).to match('test')
  end

  it 'can open-uri files' do
    FileUtils.mkdir_p '/tmp'
    File.open('/tmp/a.b', 'w+') { |f| f.puts 'test' }

    begin
      open('/tmp/a.b').read
    rescue Exception => e
      puts e
      puts e.backtrace
      raise e
    end
    expect(data).to match('test')
  end
end

```

```
No such file or directory - /tmp/a.b
/Users/jacob/.rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/open-uri.rb:36:in `initialize'
/Users/jacob/.rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/open-uri.rb:36:in `open'
...
Failures:

  1) fakefs and open-uri can open-uri files
     Failure/Error: open('/tmp/a.b').read
     Errno::ENOENT:
       No such file or directory - /tmp/a.b
     # ./test_spec.rb:20:in `block (2 levels) in <top (required)>'

Finished in 0.0035 seconds
2 examples, 1 failure
```

This [issue](https://github.com/defunkt/fakefs/issues/22) is related. `Kernel#open` is monkey patched by `open-uri` but it eventually fails on the original implementation being called.