#send_file indifferent :status option

This fixes `#send_file` and `:status` behaviour.
The problem is described in issue #749.

After this you can specify status using both string and integer:

```ruby
# using string
post '/' do
  send_file 'poem.txt', :status => '422'
end

# or this way:
post '/' do
  send_file 'poem.txt', :status => 422
end

# example from the issue
post '/' do
  send_file 'poem.txt', :status => params[:status]
end
```