Handle missing SearchParameters header

If the header is omitted, it has the same effect as sending "{}", so
just default to this value and remove a lot of conditional code.
Remove special handling for invalid search headers

We don't need to handle errors specially here, because they shouldn't
happen in the normal course of events, and if they do happen they'll be
caught in production by the normal exception catching that happens in
skin.rb and reported appropriately without breaking the page.
Fix sample data in the typical_usage_test

Two of the tests used sample data which didn't have a <body> or inner
<div> element, which in turn caused several of the processors to raise
exceptions.  It seems reasonable for this not to be valid data, so fix
the tests to include these elements.