Fix uninitialized variable @dom_attributes warning

[johantell]

Background:
The line

(@dom_attributes || {}).merge(dom_id_and_class)

in ItemContainer
printed warnings for usage of unitialized instance variable.

simple-navigation/lib/simple_navigation/item_container.rb:27: warning: instance variable @dom_attributes not initialized

This change will simply fix the issue and remove the warning

Changes:

• Initialize @dom_attributes in initialize

[coveralls]

Coverage increased (+0.0002%) to 99.616% when pulling 9195cb0 on johantell:fix_warning into abadde3 on codeplant:master.

[coveralls]

Coverage increased (+0.0002%) to 99.616% when pulling 9195cb0 on johantell:fix_warning into abadde3 on codeplant:master.

[coveralls]

Coverage increased (+0.0002%) to 99.616% when pulling 9195cb0 on johantell:fix_warning into abadde3 on codeplant:master.