various compatibility fixes

play a little more nicely with some gems and code out in the wild. now we
support the following types of things during extraction and runtime:

  # nothing to extract, but now shouldn't asplode at runtime
  I18n.t(nil, default: [:key1, :key2])

  # works at runtime, and extractor finds the default
  I18n.t(:key, default: [:other_key, "here's the default"])

  # ok to look up lotsa keys, the array won't get to_s'd
  I18n.t([:key1, :key2, :key3])
v0.0.10