Fixed ruby2.0 Hash[nil] deprecated warinigs

In ruby 2.0, Hash[nil] makes following warnings.

    dynamic.rb:3: warning: wrong element type nil at 0 (expected array)
    dynamic.rb:3: warning: ignoring wrong elements is deprecated, remove them explicitly
    dynamic.rb:3: warning: this causes ArgumentError in the next release

This commit fixes the warnings.


@snatchev Could you check this pull-req? Thanks.