Investigate JRuby test failures

[radar]

JRuby is failing some of the Travis tests, but only seemingly with the default Gemfile + the Rails master gemfile: https://travis-ci.org/ruby-i18n/i18n/builds/476071556?utm_source=github_status&utm_medium=notification.

I cannot reproduce this issue locally. I would love some help to investigate this issue.

[knapo]

Hey @radar!

I did a quick check and found out that under a certain jruby builds (even not versions) hash or Hash is weirdly calculated - it has the same value for different Hashes.

# puts "I18n.cache_key_digest #{key.class} #{key.inspect} #{key.hash}"
I18n.cache_key_digest  Hash {:foo=>1, :bar=>2} -2017438026604914721
I18n.cache_key_digest  Hash {:foo=>2, :bar=>1} -2017438026604914721

I think it's safe to always calculate hash from string, and I believe it was the initial idea behind this. We could jsonize keys, but it's a bit overkill I think.

Also, there were a random failures, which (I think) were related to the fact that the cache was not cleared after each test case.

I'll submit a PR in a while.

[radar]

Thank you very much for looking into this @knapo!