updated the page method, added args to AR page method

In my fork its possible to set a default_page as a argument for the page method. If param[:page] is invalid, the page method would return the default_page and is not raising an exception, like it is now for the issue https://github.com/mislav/will_paginate/issues/271

```ruby
params[:page] = 'foobar'
posts = Post.where(:published => true).page(params[:page])
#posts.current_page = 1

# set default_page if necessary 
posts = Post.where(:published => true).page(params[:page], :default_page => 10)
#posts.current_page = 10
# this raises no exception, it trys to return the default_page
```


with best regards