UDTs of type map[string]interface{} errors out on marshalUDT when missing fields

[aravindvs]

This is very similar to #757. With that issue resolved, unmarshalUDT never panics. But CQL spec states this (as mentioned on the other commit as well):
"
7. User Defined Types
...
A UDT value will generally have one value
for each field of the type it represents, but it is allowed to have less values than
the type has fields.

Within a user-defined type value, all data types should use the v3 protocol
serialization format.
"
Now according to the aforementioned spec, if a UDT is of type map[string]interface{}, each "key" in the map is a field
and shouldn't we be behaving the same way like we are missing a field?

Right now gocql throws an error. I added a simple test case to illustrate this. I ran the test as follows and it should fail on top of master:
$go test -tags=integration -proto=4 -run "UDT" -v .

...

=== RUN   TestUDT_UpdateFieldMissingInterface

--- FAIL: TestUDT_UpdateFieldMissingInterface (0.16s)

udt_test.go:451: missing UDT field in map: data_v2

...

Is this behavior intentional? If so, why? Shouldn't this be treated like any other missing field in a UDT?

func TestUDT_UpdateFieldMissingInterface(t *testing.T) {
        if *flagProto < protoVersion3 {
                t.Skip("UDT are only available on protocol >= 3")
        }

        // This test tries to simulate an update to an existing UDT
        // of type map[string]interface{}
        // When we try to update an altered type with a missing key
        // in the map, we get an error, when we probably should treat it with a
        // nil value.

        updateFieldUDTMap := make(map[string]interface{})
        updateFieldUDTMap["data_v1"] = "old_version"

        session := createSession(t)
        defer session.Close()

        // 1. Create a UDT with just 2 fields
        err := createTable(session, `CREATE TYPE gocql_test.update_field_udt_map(
                data_v1 text);`)
        if err != nil {
                t.Fatal(err)
        }

        // 2.  Create a table with the above UDT as a map
        err = createTable(session, `CREATE TABLE gocql_test.update_field_map(
        id uuid,
        udt_col frozen<update_field_udt_map>,
        primary key(id));`)
        if err != nil {
                t.Fatal(err)
        }

        // 3. ALTER UDT to add a new field
        if err := createTable(session, `ALTER TYPE gocql_test.update_field_udt_map ADD data_v2 text;`); err != nil {
                t.Fatal(err)
        }
        
        // 4. Now try to Insert into the table, a UDT without the new key we "altered" above
        id := TimeUUID()
        err = session.Query("INSERT INTO update_field_map(id, udt_col) VALUES(?, ?)", id, updateFieldUDTMap).Exec()
        if err != nil {
                t.Fatal(err)
        }
}