Ensure map decoder handles non-primitive strings

GODRIVER-719

Change-Id: Id3d57e5bb8a5eb09bb18684bfe962735ef597ee6

Author: skriptble

bson/bsoncodec/default_value_decoders.go

@@ -679,6 +679,7 @@ func (dvd DefaultValueDecoders) MapDecodeValue(dc DecodeContext, vr bsonrw.Value
 		dc.Ancestor = val.Type()
 	}
 
+	keyType := val.Type().Key()
 	for {
 		key, vr, err := dr.ReadElement()
 		if err == bsonrw.ErrEOD {
@@ -695,7 +696,7 @@ func (dvd DefaultValueDecoders) MapDecodeValue(dc DecodeContext, vr bsonrw.Value
 			return err
 		}
 
-		val.SetMapIndex(reflect.ValueOf(key), elem)
+		val.SetMapIndex(reflect.ValueOf(key).Convert(keyType), elem)
 	}
 	return nil
 }

bson/bsoncodec/default_value_decoders_test.go

@@ -2129,6 +2129,12 @@ func TestDefaultValueDecoders(t *testing.T) {
 				}),
 				nil,
 			},
+			{
+				"map[mystring]interface{}",
+				map[mystring]interface{}{"pi": 3.14159},
+				buildDocument(bsoncore.AppendDoubleElement(nil, "pi", 3.14159)),
+				nil,
+			},
 			{
 				"-",
 				struct {