Allow ordering by field symbol (regression)

Fixes regression introduced by fix for Issue #75 in commit
c4a53de89c6d1886aa7960afd34e715dda080393.  It wasn't documented or
tested, but previously we supported using a field symbol for the "order"
option of search_for.  This commit adds that capability back (and a test).
Merge pull request #80 from rpaterson/order_symbol

Allow ordering by field symbol (regression)