Fix route_param documentation and type

[4ndv]

Grape::Entity#merge_params returns route.params, which effectively discards result of code above this line, because route.params produces new object each time it's called

Fixes #942 and (potentially) #325

Side note: I think filter condition in delete_if at least requires an explanation comment/revision, because it's a little bit cryptic

[dblock]

Looks right, thanks for fixing this!