Fixes #64: fix crash when pressing Ctrl-r Ctrl-r

Under Ruby 1.9, pressing Ctrl-r or any of the other special search keys
while already performing an interactive search would raise an exeption.

Tested-by: Mark Somerville <mark@scottishclimbs.com> [F17, MRI 1.9.3/1.8.7]
Signed-off-by: Mark Somerville <mark@scottishclimbs.com>

lib/rbreadline.rb

@@ -3973,7 +3973,7 @@ def _rl_isearch_dispatch(cxt, c)
     #   variable isearch-terminators) are used to terminate the search but
     #   not subsequently execute the character as a command.  The default
     #   value is "\033\012" (ESC and C-J).
-    if (cxt.search_terminators.include?(cxt.lastc))
+    if (cxt.search_terminators.include?(cxt.lastc.to_s))
       # ESC still terminates the search, but if there is pending
       #input or if input arrives within 0.1 seconds (on systems
       #with select(2)) it is used as a prefix character