bugfix: groupBy transducer functionality

Fixes ramda#3232
Reverts code change in #9b5d8925

Setting an absent absent/nullish accumulator, rather than pasing an
empty on to reduceBy, appears to correct the cached copy behavior
causing pollution in the accumulators on subsequent invocations, as well
as the "passthrough"; I believe this is due due the cached
shallow-copying combined with the currying of `reduceBy`.

It may be more desirable to correct the copy caching to prevent the
error while still deterministically setting the accumulator, but for
what it's worth, it's at least in line with traditional transducer
behavior to get the initial value from the transformer, albeit not
*quite* this way!

source/groupBy.js

@@ -44,7 +44,10 @@ import reduceBy from './reduceBy.js';
  *      // }
  */
 var groupBy = _curry2(_checkForMethod('groupBy', reduceBy(function(acc, item) {
+  if (acc == null) {
+    acc = [];
+  }
   acc.push(item);
   return acc;
-}, [])));
+}, null)));
 export default groupBy;

test/groupBy.js

@@ -44,4 +44,12 @@ describe('groupBy', function() {
     eq(_isTransformer(R.groupBy(byType, xf)), true);
   });
 
+  it('works as a transducer without polluting state', function() {
+    eq(R.into([], R.groupBy(x => x.length), ['a', 'xyz', 'ab', 'xy', 'abc', 'x']), [
+      [1, ['a', 'x']],
+      [2, ['ab', 'xy']],
+      [3, ['xyz', 'abc']]
+    ]);
+    eq(R.groupBy(x => x.length, ['a', 'xyz', 'ab', 'xy', 'abc', 'x']), {1: ['a', 'x'], 2: ['ab', 'xy'], 3: ['xyz', 'abc']});
+  });
 });