[SPARK-20098][PYSPARK] dataType's typeName fix

## What changes were proposed in this pull request?
`typeName`  classmethod has been fixed by using type -> typeName map.

## How was this patch tested?
local build

Author: Peter Szalai <szalaipeti.vagyok@gmail.com>

Closes #17435 from szalai1/datatype-gettype-fix.

python/pyspark/sql/tests.py

@@ -209,6 +209,10 @@ def test_empty_row(self):
         row = Row()
         self.assertEqual(len(row), 0)
 
+    def test_struct_field_type_name(self):
+        struct_field = StructField("a", IntegerType())
+        self.assertRaises(TypeError, struct_field.typeName)
+
 
 class SQLTests(ReusedPySparkTestCase):
 

python/pyspark/sql/types.py

@@ -440,6 +440,11 @@ def toInternal(self, obj):
     def fromInternal(self, obj):
         return self.dataType.fromInternal(obj)
 
+    def typeName(self):
+        raise TypeError(
+            "StructField does not have typeName. "
+            "Use typeName on its type explicitly instead.")
+
 
 class StructType(DataType):
     """Struct type, consisting of a list of :class:`StructField`.