Derive PartialEq for Update structs

To allow easy checks for "empty" updates, this change adds the PartialEq
derive to the generated Update structs. This allows comparison with
`Default::default()` to check if an update is empty.

Fixes: #117

src/code.rs

@@ -125,6 +125,7 @@ pub mod derives {
     pub const SELECTABLE: &str = "Selectable";
     pub const IDENTIFIABLE: &str = "Identifiable";
     pub const ASSOCIATIONS: &str = "Associations";
+    pub const PARTIALEQ: &str = "PartialEq";
 }
 
 impl<'a> Struct<'a> {
@@ -200,6 +201,7 @@ impl<'a> Struct<'a> {
                     .all(|f| self.table.primary_key_column_names().contains(&f.name))
                 {
                     derives_vec.push(derives::ASCHANGESET);
+                    derives_vec.push(derives::PARTIALEQ);
                 }
 
                 derives_vec.push(derives::DEFAULT);