Setting thin config's threaded flag to false

Otherwise getting only the 1st request sent to the server is processed.
It only happens in a specific configuration (set of gems?) which has not
been determined yet. Tests couldn't incriminate a ruby version (tested
1.9.2-p290 and 1.9.3-p194), neither thin version (1.3.1 and 1.4.1 tested),
nor the nginx version used (issue happened while using thin as nginx's upstream,
and wasn't tested otherwise, but it may happen when thin is connected directly).