Added unit test for the mysql2 bind substitution

test/visitors/test_bind_visitor.rb

@@ -1,9 +1,28 @@
 require 'helper'
 require 'arel/visitors/bind_visitor'
+require 'support/fake_record'
 
 module Arel
   module Visitors
-    class TestBindVisitor < MiniTest::Unit::TestCase
+    class TestBindVisitor < MiniTest::Unit::TestCase 
+
+      ##
+      # Tests visit_Arel_Nodes_Assignment correctly
+      # substitutes binds with values from block
+      def test_assignment_binds_are_substituted
+        table = Table.new(:users)
+        um = Arel::UpdateManager.new Table.engine
+        bp = Nodes::BindParam.new '?'
+        um.set [[table[:name], bp]]
+        visitor = Class.new(Arel::Visitors::ToSql) {
+          include Arel::Visitors::BindVisitor
+        }.new Table.engine.connection
+
+        assignment = um.ast.values[0]
+        actual = visitor.accept(assignment) { "replace" } 
+        actual.must_be_like "\"name\" = replace"
+      end
+
       def test_visitor_yields_on_binds
         visitor = Class.new(Arel::Visitors::Visitor) {
           def initialize omg