Chain fallback backends

[vipera]

Building on #470, if using fnando/i18n-js and some translations are present in a Simple backend, and some in an ActiveRecord backend, the current deep_merge! done in the chain backend will override a valid translation with a nil value if a previously chained backend yields nil.

This patch updates the Hash#deep_merge! implementation to match the activesupport one, and provides a block to prevent nil values from affecting translations yielded by a chained backend.

Use case:
I'm using a chain backend of (Simple, ActiveRecord). The simple backend has an empty or partial YAML, like this:

---
en:
  test:

The activerecord backend has a record that overrides it:

> select * from translations where locale = 'en' and key = 'test';

| locale | key  | value | ...
+--------+------+-------+----
|     en | test |  Test | ...

I think the nil YAML value shouldn't prevent the other backend from yielding its value, that's how I18n::Backend::Chain#translate works, after all.

Thanks for reviewing this!

[CrAsH1101]

Any updates on this one? I would love to see this one merged ;)

[radar]

LGTM. Thank you!

[radar]

This change has now gone out in i18n 1.8.0. Thanks!