Add missing head, head(n), and first methods for TypedDatasets.

dataset/src/main/scala/frameless/TypedDataset.scala

@@ -253,6 +253,26 @@ class TypedDataset[T] protected[frameless](val dataset: Dataset[T])(implicit val
     */
   def toLocalIterator[F[_]]()(implicit F: SparkDelay[F]): F[util.Iterator[T]] =
     F.delay(dataset.toLocalIterator())
+
+  /** Alias for firstOption().
+    */
+  def headOption[F[_]]()(implicit F: SparkDelay[F]): F[Option[T]] = firstOption()
+
+  /** Alias for take().
+    */
+  def head[F[_]](num: Int)(implicit F: SparkDelay[F]): F[Seq[T]] = take(num)
+
+  // $COVERAGE-OFF$
+  /** Alias for firstOption().
+    */
+  @deprecated("Method may throw exception. Use headOption or firstOption instead.", "0.5.0")
+  def head: T = dataset.head()
+
+  /** Alias for firstOption().
+    */
+  @deprecated("Method may throw exception. Use headOption or firstOption instead.", "0.5.0")
+  def first: T = dataset.head()
+  // $COVERAGE-ONN$
 
   /** Displays the content of this [[TypedDataset]] in a tabular form. Strings more than 20 characters
     * will be truncated, and all cells will be aligned right. For example:

dataset/src/test/scala/frameless/forward/HeadTests.scala

@@ -0,0 +1,31 @@
+package frameless.forward
+
+import frameless.{TypedDataset, TypedDatasetSuite, TypedEncoder, TypedExpressionEncoder, X1}
+import org.apache.spark.sql.SparkSession
+import org.scalacheck.Prop
+import org.scalacheck.Prop._
+import org.scalatest.Matchers
+
+import scala.reflect.ClassTag
+
+class HeadTests extends TypedDatasetSuite with Matchers {
+  def propArray[A: TypedEncoder : ClassTag : Ordering](data: Vector[X1[A]])(implicit c: SparkSession): Prop = {
+    import c.implicits._
+    if(data.nonEmpty) {
+      val tds = TypedDataset.
+        create(c.createDataset(data)(
+          TypedExpressionEncoder.apply[X1[A]]
+        ).orderBy($"a".desc))
+        (tds.headOption().run().get ?= data.max).
+        &&(tds.head(1).run().head ?= data.max).
+        &&(tds.head(4).run().toVector ?=
+          data.sortBy(_.a)(implicitly[Ordering[A]].reverse).take(4))
+    } else Prop.passed
+  }
+
+  test("headOption(), head(1), and head(4)") {
+    check(propArray[Int] _)
+    check(propArray[Char] _)
+    check(propArray[String] _)
+  }
+}