Allow to write regexp in :skip Array

It allows to write things like: 

```ruby
use Rack::SimpleCsrf, {
    skip: ["/skip", "/otherskip/.*"]

}
```

The code is more verbose than before, but according to me much more clean =)

It also fixes the tests which where always true.

Explanation

```ruby
described_class.new(app, :skip => ["/"]).call(@env).should be_true
described_class.new(app, :skip => ["/"]).call(@env).should be([403, {}, ["Unauthorized"]])
```

are both true because `[403, {}, ["Unauthorized"]]` is considered as `true` by `be_true`, that's why
I've switched it to `TrueClass`