Add page part now works again, but test still failing.

When page_part.content was nil, as it is on a new part, ck_editor tak failed. Now passing an empty string if this is nil.

Issue #132
Tests seem to be fixed to input text into CKEditor fields.

I'm not quite familiar enough with Capybara's execute_script and evaluate_script methods (and documentation on them is pretty thin) to know if I've done something Not The Way I Should, but this seems to be working now. @carols10cents, if you have a moment (since you know Capybara better than I do) could you take a look at these changes and see if I'm in the right vicinity of how to do this?

Issue #132