Add fp tests for placeholders.

test/test-fp.js

@@ -510,6 +510,24 @@
 
   QUnit.module('placeholder methods');
 
+  (function() {
+    QUnit.test('should support placeholders', function(assert) {
+      assert.expect(6);
+
+      _.each([[], fp.__], function(ph) {
+        fp.placeholder = ph;
+
+        var actual = fp.add(ph, 'b')('a');
+        assert.strictEqual(actual, 'ab');
+
+        actual = fp.slice(ph, 2)(1)(['a', 'b', 'c']);
+        assert.deepEqual(actual, ['b']);
+
+        actual = fp.fill(ph, 2)(1, '*')([1, 2, 3]);
+        assert.deepEqual(actual, [1, '*', 3]);
+      });
+    });
+
     _.forOwn(mapping.placeholder, function(truthy, methodName) {
       var func = fp[methodName];
 
@@ -520,6 +538,7 @@
         assert.strictEqual(func.placeholder, fp.__);
       });
     });
+  }());
 
   /*--------------------------------------------------------------------------*/