Fix .belongs_to_static to work with ActiveSupport 3

ActiveSupport 3 String#classify does not return the correct singular
class name for strings ending in 'ss'.

Examples:
* business => Busines
* dummy_class => DummyClas

Use String#camelize to resolve this.

This is not an issue in ActiveSupport 4 as it does not change strings
ending in 'ss'.
Merge pull request #1 from New-Bamboo/fix_belongs_to_static_with_active_support_3

Fix .belongs_to_static to work with ActiveSupport 3