Fix issue where /about fields default to false

The issue was the all fields were getting cast to String values. When
the page was saved the correct settings were saved in memory, but a
string representation of the settings were saved to the database. So the
problem didn't become apparent until the process was restarted and the
settings were loaded from the database again.
Merge pull request #384 from mysociety/false-settings-issue

Fix issue where /about fields default to false