Better support for hash_including matching

[bdurand]

• Lumberjack::LogEntryMatcher no longer ignores an attributes filter that is not a hash. Passing a matcher like RSpec's hash_including as the entire filter silently matched every entry; it is now applied to the entry's attributes hash with ===.
• Lumberjack::LogEntryMatcher now exposes log entry attributes to matchers with indifferent key access, so matchers that do their own key lookups (i.e. RSpec's hash_including) can use either string or symbol attribute names.