Fix pluralization behavior for KeyValue backend with subtrees disabled

[wjordan]

This PR includes a fix making the pluralization behavior for the KeyValue backend with subtrees disabled conform more closely to the behavior of the default Simple backend, as well as the KeyValue backend with subtrees enabled.

My use-case for this PR is migrating an existing application from a Simple to a KeyValue backend with subtrees disabled for performance, where the existing application depends on specific fallback/pluralization semantics to run without further modification.

The simple code change to key_value.rb fixes the following two errors, which I've reproduced in the added tests:

  1) Error:
 I18nBackendKeyValueTest#test_subtrees_disabled:_Returns_fallback_default_given_missing_pluralization_data:
NoMethodError: undefined method `[]' for nil:NilClass
    [Code]/i18n/lib/i18n/backend/key_value.rb:131:in `pluralize'
    [Code]/i18n/lib/i18n/backend/base.rb:48:in `translate'
    [Code]/i18n/lib/i18n/backend/fallbacks.rb:48:in `block (2 levels) in translate'
    [Code]/i18n/lib/i18n/backend/fallbacks.rb:47:in `catch'
    [Code]/i18n/lib/i18n/backend/fallbacks.rb:47:in `block in translate'
    [Code]/i18n/lib/i18n/backend/fallbacks.rb:45:in `each'
    [Code]/i18n/lib/i18n/backend/fallbacks.rb:45:in `translate'
    [Code]/i18n/lib/i18n.rb:185:in `block in translate'
    [Code]/i18n/lib/i18n.rb:181:in `catch'
    [Code]/i18n/lib/i18n.rb:181:in `translate'
    [Code]/i18n/test/backend/key_value_test.rb:81:in `block in <class:I18nBackendKeyValueTest>'

  2) Error:
I18nBackendKeyValueTest#test_subtrees_disabled:_returns_localized_string_given_missing_pluralization_data:
TypeError: no implicit conversion of Symbol into Integer
    [Code]/i18n/lib/i18n/backend/key_value.rb:131:in `[]'
    [Code]/i18n/lib/i18n/backend/key_value.rb:131:in `pluralize'
    [Code]/i18n/lib/i18n/backend/base.rb:48:in `translate'
    [Code]/i18n/lib/i18n.rb:185:in `block in translate'
    [Code]/i18n/lib/i18n.rb:181:in `catch'
    [Code]/i18n/lib/i18n.rb:181:in `translate'
    [Code]/i18n/test/backend/key_value_test.rb:69:in `block in <class:I18nBackendKeyValueTest>'

[radar]

Thanks @wjordan. This LGTM. I'll merge it now and release it in a new i18n release later today.