Inserting enum inserts incorrect value when using InsertParam + Unicode

The generated SQL for the insert will be this:

```SQL
exec sp_executesql N'INSERT INTO "TypeWithEnum" ("Id","EnumValue") VALUES (@Id,@EnumValue)',N'@Id int,@EnumValue nvarchar(8)',@Id=1,@EnumValue=N'N''Value2'
```

The problem is `@EnumValue` contains a leading `N'`.
Traced it back to \src\servicestack.ormlite\ormlitedialectproviderbase.cs:631 where 
```C#
var unquotedVal = OrmLiteConfig.DialectProvider.GetQuotedValue(value, fieldDef.FieldType)
    .TrimStart('\'').TrimEnd('\''); ;
```
quotes the value (`UseUnicode == true` adds `N'` to the begining), and tries to "unquote" it, but that leaves the `N'` on the beginning.

(btw, enums default to the first possible value, so with SomeEnum.Value1, the test would succeed)