Allow lambda procs with 0 arity as virtual row blocks on ruby 1.9

Previously, such blocks did not work, because the blocks would be
instance_evaled, which passes an argument, and lambda procs with
0 arity would raise an error in that circumstance on ruby 1.9+.

Now that Sequel no longer supports ruby 1.8.6, switch from
instance_eval to instance_exec, making such procs work.

For compatibility with ruby 1.8.7, make Sequel::BasicObject
respond to instance_exec on ruby <1.9.

CHANGELOG

@@ -1,5 +1,7 @@
 === HEAD
 
+* Allow lambda procs with 0 arity as virtual row blocks on ruby 1.9 (jeremyevans)
+
 * Handle schema-qualified row_types in the pg_array integration in the pg_row extension (jeremyevans) (#595)
 
 * Support default_schema when reseting primary key sequences on PostgreSQL (jeremyevans) (#596)

lib/sequel/core.rb

@@ -390,7 +390,7 @@ def self.virtual_row(&block)
     vr = SQL::VirtualRow.new
     case block.arity
     when -1, 0
-      vr.instance_eval(&block)
+      vr.instance_exec(&block)
     else
       block.call(vr)
     end  

lib/sequel/sql.rb

@@ -6,7 +6,7 @@ module Sequel
     class BasicObject
       # The instance methods to not remove from the class when removing
       # other methods.
-      KEEP_METHODS = %w"__id__ __send__ __metaclass__ instance_eval == equal? initialize method_missing"
+      KEEP_METHODS = %w"__id__ __send__ __metaclass__ instance_eval instance_exec == equal? initialize method_missing"
 
       # Remove all but the most basic instance methods from the class.  A separate
       # method so that it can be called again if necessary if you load libraries

spec/core/expression_filters_spec.rb

@@ -594,6 +594,11 @@ def l(arg, should)
     l(Sequel.expr(proc{|v| @o}) + 1, "(foo + 1)")
   end
 
+  it "Sequel.expr should handle lambda proc virtual rows" do
+    l(Sequel.expr(&lambda{1}), "1")
+    l(Sequel.expr(&lambda{|| 1}), "1")
+  end
+
   it "Sequel.expr should raise an error if given an argument and a block" do
     proc{Sequel.expr(nil){}}.should raise_error(Sequel::Error)
   end