Prevent calling regexp on symbol in Ruby 1.9 in association_proxy

Signed-off-by: NPratik Naik <pratiknaik@gmail.com>

activerecord/lib/active_record/associations/association_proxy.rb

@@ -51,7 +51,7 @@ class AssociationProxy #:nodoc:
       alias_method :proxy_respond_to?, :respond_to?
       alias_method :proxy_extend, :extend
       delegate :to_param, :to => :proxy_target
-      instance_methods.each { |m| undef_method m unless m =~ /^(?:nil\?|send|object_id|to_a)$|^__|proxy_/ }
+      instance_methods.each { |m| undef_method m unless m.to_s =~ /^(?:nil\?|send|object_id|to_a)$|^__|proxy_/ }
 
       def initialize(owner, reflection)
         @owner, @reflection = owner, reflection