Contributions - Fix Search on Custom Contribution Fields

In 1.6 we "appear" to allow Contribution custom fields to be included in the CiviContribute search block (they show up in the search form if they are checked "Searchable"). However, if you try to search on a custom contribution field, the query throws a fatal error. Example from test on demo —

Unknown column 'custom_value_3.char_data' in 'where clause',

For 1.7 - let's make sure this is working for Contribution custom fields.

We'll extend things in 1.8 to handle Member, Event, Activity custom fields. If it's not too hard, we should suppress any custom fields for these object types from appearing in the search blocks in 1.7.