follow standard theory merge behaviour: do not change already selected solver

--- a/src/HOL/SMT/Tools/smt_solver.ML	Mon Nov 09 11:19:25 2009 +0100
+++ b/src/HOL/SMT/Tools/smt_solver.ML	Mon Nov 09 11:34:22 2009 +0100
@@ -227,7 +227,7 @@
   type T = string
   val empty = no_solver
   val extend = I
-  fun merge (_, s) = s
+  fun merge (s, _) = s
 )
 
 val solver_name_of = Selected_Solver.get