bugfix: groupBy transducer functionality

Fixes ramda#3232
Reverts code change in #9b5d8925

Setting the accumulator when not
prsent rather than passing it tin to reducerby; this prevents the cached
copy behavior causing polution of accumulators in subsequent invocations.

It may be more desirable to correct the copy caching to prevent the
error while still deterministically setting the accumulator; but it's in
line with traditional transducer behavior to get the initial accumulator
value from the transformer, albeit not quite in this fashion.

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']});
+  });
 });