fix problem with enum type in generating diff

I have a table like this :  
```
<table name="test" >
   <vendor type="mysql">
       <parameter name="Engine" value="InnoDB"/>
       <parameter name="Charset" value="utf8"/>
   </vendor>
   <column name="status" type="enum" valueSet="a,b,c" description="some desc"/>
</table>
```
diff task always include this table in migration class (and there is always a migration class) 
The problem is, In PropelColumnComparator::compareColumns from domain is some thing like this : 
```
Domain Object
(
    [name:Domain:private] => 
    [description:Domain:private] => 
    [size:Domain:private] => 
    [scale:Domain:private] => 
    [propelType:Domain:private] => TINYINT
    [sqlType:Domain:private] => TINYINT
    [defaultValue:Domain:private] => 
    [database:Domain:private] => 
    [attributes:protected] => Array
        (
        )

    [vendorInfos:protected] => Array
        (
        )

)
```
But toDomain is like this : 
```
Domain Object
(
    [name:Domain:private] => 
    [description:Domain:private] => 
    [size:Domain:private] => 
    [scale:Domain:private] => 
    [propelType:Domain:private] => ENUM
    [sqlType:Domain:private] => TINYINT
    [defaultValue:Domain:private] => 
    [database:Domain:private] => 
    [attributes:protected] => Array
        (
        )

    [vendorInfos:protected] => Array
        (
        )

)
```
The propelType in From field is TINYINT but in to ENUM  (its default in propel as I know) so the compare always return this as diff. 

This also happen when there is a default value in enum fields.