Fix using multi-star scope with top-level translation key

Given the following locales.yml

:en
  foo: 'Foo'

and the following scope: "*.*.formats"

this error is raised:

Failures:

  1) I18n::JS filters filters translations using multi-star scope
     Failure/Error: result = I18n::JS.scoped_translations("*.*.formats")
     NoMethodError:
       undefined method `has_key?' for "Foo":String
     # ./lib/i18n/js.rb:134:in `filter'
     # ./lib/i18n/js.rb:130:in `block in filter'
     # ./lib/i18n/js.rb:127:in `each'
     # ./lib/i18n/js.rb:127:in
     # `filter'
     # ./lib/i18n/js.rb:130:in
     # `block in filter'
     # ./lib/i18n/js.rb:127:in `each'
     # ./lib/i18n/js.rb:127:in
     # `filter'
     # ./lib/i18n/js.rb:113:in `block in scoped_translations'
     # ./lib/i18n/js.rb:112:in
     # `each'
     # ./lib/i18n/js.rb:112:in `scoped_translations'
     # ./spec/i18n_js_spec.rb:82:in
     # `block (3 levels) in <top (required)>''`

This commit will fix it by type-checking.