Fix UDF returning StructType when it is the only column.

[imback82]

This is a follow up to #376 to address the following issue:

var schema = new StructType(new[] {
    new StructField("col1", new IntegerType()),
    new StructField("col2", new StringType()) });
    
var udf = Udf<string>(str => new GenericRow(new object[] { 1, "abc" }), schema);

Row[] rows = _df.Select(udf(_df["name"]).As("col")).Collect().ToArray();
Assert.Equal(3, rows.Length);
rows[0].GetAs<Row>("col"); // This would fail!

[suhsteve]

Looks like withcolumn tests are failing.

[elvaliuliuliu]

Tests are failing.

[imback82]

@elvaliuliuliu will create a separate PR for this.