Allows passing options through has_checked_field? and has_unchecked_field? to has_selector?

This enables assertions on the presence of disabled checked fields by doing:

    assert has_checked_field?('Disabled Checkbox', :disabled => true)

in a similar way to:

    find_field("Disabled Checkbox", :disabled => true)
Merge pull request #1140 from carols10cents/has_checked_field_options

Allows passing options through has_checked_field? and has_unchecked_field? to has_selector?