allow for configuration keys with nil values to fall back onto defaults

[cmw]

In our effort to turn our deployment into something 12-factor-compliant, we tried to base our configuration solely on ENV variables like so:

url: <%= ENV['REDIS_URL'].presence || nil %>
sentinels:
  - host: <%= ENV['REDIS_SENTINEL_HOST'].presence || nil %>
    port: <%= ENV['REDIS_SENTINEL_PORT'].presence || nil %>
role: <%= ENV['REDIS_ROLE'].presence || nil %>
timeout: <%= ENV['REDIS_TIMEOUT'].presence || 0.1 %>
db: <%= ENV['REDIS_DB'].presence || 0 %>
host: <%= ENV['REDIS_HOST'].presence || '127.0.0.1' %>
port: <%= ENV['REDIS_PORT'].presence || 6379 %>

This however results in the configuration hash holding several unnecessary keys with nil values. Thus these changes are needed so in case of a key existing but being nil, the default is still used.

I assume this was intended all along, but did only work for some of the keys, not all.

[cmw]

@badboy is there a chance to get this merged?