Adds support for properly logging Arrays

[KevinMcHugh]

Hello! Thank you for providing this gem. I noticed a possible issue in my team's usage of it. Without this patch, an Array will be flattened, resulting in keys and values becoming mismatched.

Eg, given a request that responds to some_array_method with [1,2]:

logger = Fluent::Logger::FluentLogger.new
logger.tagged(array: :some_array_method, date: Date.today)

You might expect to see log entries having tags like:

{ array: [1,2], date: '2021-07-20' }

but it will actually have tags like:

{ array: 1, date: 2}

This can be worked around by passing a proc which calls to_s on any array values,
but it seems unlikely users would expect their keys to map to different values if some method returns an Array.