Added respond_to block for export action

Fixes Safari, IE adding .html extension to csv exports due to incorrect content type. The content type was set by manually adding response headers, but those are overridden
Fix columns being formatted with Rails humanize helper h(), resulting in html escaping in csv export

ActiveScaffoldExport calls format_value from AS which in turn calls clean_column_value, which calles h(value), improperly escaping HTML chars in CSV export.
Removed unused options param
Merge pull request #6 from nextlot/master

merges from nextlot, thanks mtarnovan