[FEAT] Make sure fetch also works as expected in with_indifferent behavior

I understand sinatra wouldn't want to pull in ActiveSupport, makes perfect sense.

However, I came across a real surprising issue trying to debug the behaviour around the poor men's implementation of with_indifferent_access.
Given the following param:
````ruby
params = {"host"=>"subcategory"}
`````
I see:
````ruby
params.class # => 'Hash'
params['host'] = "subcategory"
params[:host] = "subcategory"
````
All good so far, however when I try to use ````fetch````, I see the following unexpected behaviour:

````ruby
params.fetch(:host, 'default') # => 'default'
````


Updating only the params eigen class should fix that with little risk to this world, and beyond.