In Elasticsearch 5.3.0 a bug was introduced in the merging of default
settings when the target setting existed as an array. This arose due to
the fact that when a target setting is an array, the setting key is
broken into key.0, key.1, ..., key.n, one for each element of the
array. When settings are replaced by default.key, we are looking for the
target key but not the target key.0. This leads to key, and key.0, ...,
key.n being present in the constructed settings object. This commit
addresses two issues here. The first is that we fix the merging of the
keys so that when we try to merge default.key, we also check for the
presence of the flattened keys. The second is that when we try to get a
setting value as an array from a settings object, we check whether or
not the backing map contains the top-level key as well as the flattened
keys. This latter check would have caught the first bug. For kicks, we
add some tests.

* master:
  Remove more hidden file leniency from plugins
  Register error listener in evil logger tests
  Detect using logging before configuration
  [DOCS] Added note about Elastic Cloud to improve 'elastic aws' SERP results.
  Add version constant for 5.5 (elastic#24075)
  Add unit tests for NestedAggregator (elastic#24054)