minidom: ensure prefix is extracted out of provided name when creating Element

I would have liked to handle all of this in `Element::new` only, but I
also have to do it in `Element::builder` unfortunately because then
element builder then pushes prefixes it gathered itself.

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

minidom-rs/src/prefixes.rs

@@ -52,6 +52,10 @@ impl Prefixes {
             None => None,
         }
     }
+
+    pub(crate) fn insert<S: Into<String>>(&mut self, namespace: S, prefix: Option<String>) {
+        self.prefixes.insert(namespace.into(), prefix);
+    }
 }
 
 impl From<BTreeMap<String, Option<String>>> for Prefixes {