- Switch from Array#include? to Range#cover? for more than 5 items

fix issues with RDoc generation

Similarly to #599, I've observed issues issues where untrusted user input
that includes interpolation patterns gets unintentionally interpolated
and leads to bogus `I18n::MissingInterpolationArgument` exceptions.

This happens when multiple lookups are required for a key to be resolved,
which is common when resolving defaults, or resolving a key that itself
resolves to a Symbol.

As an example let's consider these translations, common for Rails apps:

```yaml
en:
  activerecord:
    errors:
      messages:
        taken: "%{value} has already been taken"
```

If the `value` given to interpolate ends up containing interpolation characters,
and Rails specifies default keys (as [described here](https://guides.rubyonrails.org/i18n.html#error-message-scopes)), resolving
those defaults will cause a `I18n::MissingInterpolationArgument` to be raised:

```rb
I18n.t('activerecord.errors.models.organization.attributes.name.taken',
  value: '%{dont_interpolate_me}',
  default: [
    :"activerecord.errors.models.organization.taken",
    :"activerecord.errors.messages.taken"
  ]
)
```

Raises:

```
I18n::MissingInterpolationArgument: missing interpolation argument :dont_interpolate_me in "%{dont_interpolate_me}" ({:value=>"%{dont_interpolate_me}"} given)
```

Instead of this, we'd expect the translation to resolve to:

```
%{dont_interpolate_me} has already been taken
```

This behaviour is caused by the way that recursive lookups work: whenever a
key can't be resolved to a string directly, the `I18n.translate`
method is called either to walk through the defaults specified, or if a Symbol
is matched, to try to resolve that symbol.

This results in interpolation being executed twice for recursive
lookups... once on the pass that finally resolves to a string, and again
on the original call to `I18n.translate`.

A "proper" fix here would likely revolve around decoupling key resolution
from interpolation... it feels odd to me that the `resolve_entry` method calls
`I18n.translate`... however I see this as a fundamental change beyond
the scope of this fix.

Instead I'm proposing to add a new reserved key `skip_interpolation`
that gets passed down into every recursive call of `I18n.translate` and
instructs the method to skip interpolation.

This ensures that only the initial `I18n.translate` call is the one that
gets its string interpolated.

… is a Pathname

Fix loading of .rb locale files when `load_path` is not a string

Fixes strings being interpolated multiple times

Optimize pluralization logic in test data

[FIX] Raise ArgumentError on nil key in exists?

As per #686