use src map if dst is nil and can't be set

see darccio#90

Signed-off-by: Nicolas De Loof <nicolas.deloof@gmail.com>

go.mod

@@ -1,4 +1,4 @@
-module github.com/imdario/mergo
+module github.com/ndeloof/mergo
 
 go 1.13
 

issue90_test.go

@@ -0,0 +1,42 @@
+package mergo
+
+import (
+	"reflect"
+	"testing"
+)
+
+type structWithMap struct {
+	Data map[string]string
+}
+
+
+func TestIssue90(t *testing.T) {
+    dst := map[string]structWithMap{
+    	"struct": {
+    		Data: nil,
+		},
+	}
+	src := map[string]structWithMap{
+		"struct": {
+			Data: map[string]string{
+				"foo": "bar",
+			},
+		},
+	}
+	expected := map[string]structWithMap{
+		"struct": {
+			Data: map[string]string{
+				"foo": "bar",
+			},
+		},
+	}
+
+	err := Merge(&dst, src, WithOverride)
+	if err != nil {
+		t.Errorf("unexpected error %v", err)
+	}
+
+	if !reflect.DeepEqual(dst, expected) {
+		t.Errorf("expected: %#v\ngot: %#v", expected, dst)
+	}
+}

merge.go

@@ -101,7 +101,12 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 		}
 	case reflect.Map:
 		if dst.IsNil() && !src.IsNil() {
-			dst.Set(reflect.MakeMap(dst.Type()))
+			if dst.CanSet() {
+				dst.Set(reflect.MakeMap(dst.Type()))
+			} else {
+				dst = src
+				return
+			}
 		}
 
 		if src.Kind() != reflect.Map {