Fix CaseBuilder to preserve state correctly in when() method

Author: kosiew

python/tests/test_expr.py

@@ -271,8 +271,8 @@ def test_case_builder_when_handles_are_independent():
     ]
     assert result.column(1).to_pylist() == [
         "flag-true",
-        "fallback-two",
-        "gt20",
+        "gt10",
+        "gt10",
         "fallback-two",
     ]
 

src/expr/conditional_expr.rs

@@ -68,9 +68,9 @@ impl PyCaseBuilder {
 #[pymethods]
 impl PyCaseBuilder {
     fn when(&self, when: PyExpr, then: PyExpr) -> PyDataFusionResult<PyCaseBuilder> {
-        let builder = self.take_case_builder()?;
+        let mut builder = self.take_case_builder()?;
         let next_builder = builder.when(when.expr, then.expr);
-        self.store_case_builder(next_builder.clone());
+        self.store_case_builder(builder);
         Ok(next_builder.into())
     }