Warn when using #eql? on zero amounts with different currencies

[sunny]

Closes #1122

With this change comparing two zero amounts with different currencies using #eql? triggers a warning:

Money.new(1, "USD").eql?(Money.new(1, "EUR")) #=> false
Money.new(0, "USD").eql?(Money.new(0, "EUR")) #=> true
#> [DEPRECATION] Comparing 0 USD with 0 EUR using `#eql?` will return false in future versions of Money. Opt-in to the new behavior by setting `Money.strict_eql_compare = true`.

To disable this warning, opt-in to the new behavior so that it always does a strict comparison:

Money.strict_eql_compare = true
Money.new(1, "USD").eql?(Money.new(1, "EUR")) #=> false
Money.new(0, "USD").eql?(Money.new(0, "EUR")) #=> false

I’m guessing that using a configuration to opt-in can help ease into this new behavior, but I’m not sure this would be the best way to introduce this both a warning and a config. Happy for feedback about this.

[yukideluxe]

love this! thanks!