update purchase_date type

git-svn-id: http://svn.code.sf.net/p/itop/code/trunk@2426 a333f486-631f-4898-b8df-5754b55c2be0

datamodels/2.x/itop-config-mgmt/datamodel.itop-config-mgmt.xml

@@ -1541,7 +1541,7 @@
           <default_value/>
           <is_null_allowed>true</is_null_allowed>
         </field>
-        <field id="purchase_date" xsi:type="AttributeString">
+        <field id="purchase_date" xsi:type="AttributeDate">
           <sql>purchase_date</sql>
           <default_value/>
           <is_null_allowed>true</is_null_allowed>