handle map-merge key overlaps

src/value/map.rs

@@ -1,4 +1,5 @@
 use std::slice::Iter;
+use std::vec::IntoIter;
 
 use super::Value;
 use crate::error::SassResult;
@@ -30,7 +31,9 @@ impl SassMap {
     }
 
     pub fn merge(&mut self, other: SassMap) {
-        self.0.extend(other.0);
+        for (key, value) in other.into_iter() {
+            self.insert(key, value);
+        }
     }
 
     pub fn iter(&self) -> Iter<(Value, Value)> {
@@ -55,3 +58,12 @@ impl SassMap {
         self.0.push((key, value));
     }
 }
+
+impl IntoIterator for SassMap {
+    type Item = (Value, Value);
+    type IntoIter = IntoIter<Self::Item>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.into_iter()
+    }
+}

tests/map.rs

@@ -78,6 +78,11 @@ test!(
     "a {\n  color: inspect(map-merge((), ()));\n}\n",
     "a {\n  color: ();\n}\n"
 );
+test!(
+    map_merge_same_keys,
+    "a {\n  color: inspect(map-merge((c: d, e: f), (c: 1, e: 2)));\n}\n",
+    "a {\n  color: (c: 1, e: 2);\n}\n"
+);
 error!(
     map_merge_map1_non_map,
     "a {\n  color: map-merge(foo, (a: b));\n}\n", "Error: $map1: foo is not a map."