fix(transform): don't duplicate transform steps on save

this was caused by a bug in dataset.Transform.Assign

go.mod

@@ -42,7 +42,7 @@ require (
 	github.com/olekukonko/tablewriter v0.0.4
 	github.com/pkg/errors v0.9.1
 	github.com/qri-io/dag v0.2.2-0.20201208212257-ae00241c4b48
-	github.com/qri-io/dataset v0.2.1-0.20210312210644-ba8eaa336c8d
+	github.com/qri-io/dataset v0.2.1-0.20210317143526-3e9020e68b0e
 	github.com/qri-io/deepdiff v0.2.1-0.20200807143746-d02d9f531f5b
 	github.com/qri-io/didmod v0.0.0-20201123165422-8b2e224c993a
 	github.com/qri-io/doggos v0.1.0

go.sum

@@ -1131,8 +1131,8 @@ github.com/qri-io/compare v0.1.0 h1:A/MRx3uEnJ/iMjfJY1VOqH9CYs9zFSEYaFVeXuGfmis=
 github.com/qri-io/compare v0.1.0/go.mod h1:i/tVuDGRXVxhuZ8ZUieF23u6rQ6wLGJl7KKWpoMRaTE=
 github.com/qri-io/dag v0.2.2-0.20201208212257-ae00241c4b48 h1:6fTW2iHGbaEKQt9u8+04kB3m33KSGLqxF/2pWNleeEg=
 github.com/qri-io/dag v0.2.2-0.20201208212257-ae00241c4b48/go.mod h1:1AwOy3yhcZTAXzaF4wGSdnrp87u3PBOrsWXUjOtQCXo=
-github.com/qri-io/dataset v0.2.1-0.20210312210644-ba8eaa336c8d h1:5KfPirdkABg/R3/8S9xwNZUDuwBj32BHrw/BgQ9DiXw=
-github.com/qri-io/dataset v0.2.1-0.20210312210644-ba8eaa336c8d/go.mod h1:vlq9+Nu37koO3mrp25QGNOt68CLe2d2rAtB9cnDLV6E=
+github.com/qri-io/dataset v0.2.1-0.20210317143526-3e9020e68b0e h1:3C4CKHMMoOc9Ovw1qMcxXx1/Gwq1MPIplMSRk5XS+xY=
+github.com/qri-io/dataset v0.2.1-0.20210317143526-3e9020e68b0e/go.mod h1:vlq9+Nu37koO3mrp25QGNOt68CLe2d2rAtB9cnDLV6E=
 github.com/qri-io/deepdiff v0.2.1-0.20200807143746-d02d9f531f5b h1:T8qEIv+qLi5mVWvSS329wJ+HbN7cfMwCWjRVzh/+upo=
 github.com/qri-io/deepdiff v0.2.1-0.20200807143746-d02d9f531f5b/go.mod h1:NrL/b7YvexgpGb4HEO3Rlx5RrMLDfxuKDf/XDAq5ac0=
 github.com/qri-io/didmod v0.0.0-20201123165422-8b2e224c993a h1:40BIa59lae2xZ7iieb3UU4/X57jZsWZ6QgqwdjDQhig=