FIX Make column query not distinct

When selecting a column, it doesn't  make sense to have it distinct.

As an alternative, the API could be changed to give the end user the option
(eg `->column($field, $distinct = false)`)
However if we did that, we would need to make sure we do something
similar with `SilverStripe\ORM\UnsavedRelationList` to ensure consistency.

src/ORM/DataQuery.php

@@ -1147,6 +1147,7 @@ class DataQuery
     {
         $fieldExpression = $this->expressionForField($field);
         $query = $this->getFinalisedQuery(array($field));
+        $query->setDistinct(false);
         $originalSelect = $query->getSelect();
         $query->setSelect(array());
         $query->selectField($fieldExpression, $field);

tests/php/ORM/DataQueryTest.php

@@ -379,4 +379,22 @@ class DataQueryTest extends SapphireTest
         $this->assertEquals('Last', $second['Title']);
         $this->assertEmpty(array_shift($arrayResult));
     }
+
+    public function testColumnReturnsAllValues()
+    {
+        $first = new DataQueryTest\ObjectA();
+        $first->Name = 'Bar';
+        $first->write();
+
+        $second = new DataQueryTest\ObjectA();
+        $second->Name = 'Foo';
+        $second->write();
+
+        $third = new DataQueryTest\ObjectA();
+        $third->Name = 'Bar';
+        $third->write();
+
+        $result = DataQueryTest\ObjectA::get()->column('Name');
+        $this->assertEquals(['Bar', 'Foo', 'Bar'], $result);
+    }
 }