codegen: Properly detect bindgen unions.

The root cause of the issue was the missing:

```
  self.saw_bindgen_union |= new.saw_bindgen_union;
```

But I also cleaned up a bit more and removed the unused saw_union.

Fixes #1457

1 files changed, 8 insertions, 0 deletions

diff --git a/tests/headers/bindgen-union-inside-namespace.hpp b/tests/headers/bindgen-union-inside-namespace.hpp
new file mode 100644
index 00000000..6a7d3a30
--- /dev/null
+++ b/tests/headers/bindgen-union-inside-namespace.hpp
@@ -0,0 +1,8 @@
+// bindgen-flags: --rust-target 1.0 --enable-cxx-namespaces
+
+namespace foo {
+  union Bar {
+    int foo;
+    int bar;
+  };
+}