fix bug when validating within array parameters

[asross]

#333 introduced functionality to validate arrays. The case it was tested against is this:

params do
  group :tags do
    requires :name
  end
end

{ tags: [name: "ruby", name: "python"] } will pass validation, and { tags: [foo: "bar"] } will fail with a 400.

However, if you have a more complicated case, where you have new parameter scopes within each element of the array, grape throws an ArgumentError because ParamsScope#params expects to be receiving a hash (and #should_validate? also behaves unexpectedly). E.g.:

params do
  group :cats do
     requires :name
     group :owner do
        requires :name
     end
  end
end

You will get a 500 rather than a validation error or success when you try to submit:

{ 
  cats: [
    { name: 'Garfield', owner: { name: 'Arbuckle' } }, 
    { ...more cats... }
  ]
}

This pull request fixes that issue (although I'm not entirely happy with the deep nesting in #params) and, more importantly, adds a bunch of tests.

[dblock]

Check Travis, there're a few Rubocop violations (style). Update CHANGELOG. Amend/squash the PR with the changes, please. Thx.

[dblock]

Merged via 3ce41ef.