Check for mass assign in AssociationCollection#build

closes #44

lib/brakeman/checks/check_mass_assignment.rb

@@ -30,7 +30,8 @@ class Brakeman::CheckMassAssignment < Brakeman::BaseCheck
       :update_attributes, 
       :update_attributes!,
       :create,
-      :create!]
+      :create!,
+      :build]
 
     Brakeman.debug "Processing possible mass assignment calls"
     calls.each do |result|

test/tests/test_rails3.rb

@@ -15,7 +15,7 @@ class Rails3Tests < Test::Unit::TestCase
       :controller => 1,
       :model => 5,
       :template => 18,
-      :warning => 21
+      :warning => 22
     }
   end