fix(test): updated tests and docs

docs/index.md

@@ -297,7 +297,7 @@ nums.update([2, 4, 6]);  // → SuperSet { 0, 1, 2, 4, 6 }
 
 ## `discard(iterable)`
 
-The discard() method deletes the iterable elements from the set and return the updated elements.
+The discard() method deletes the iterable elements from the data set and returns the updated elements.
 
 ### Example
 

test/superset.spec.js

@@ -219,5 +219,19 @@ describe("SuperSet", () => {
 
             expect(Array.from(result)).to.eql([2]);
         });
+
+        it("should delete undefined elements and return updated elements.", () => {
+            const result = testSet.discard([7]);
+
+            expect(Array.from(result)).to.eql([1, 2, 3]);
+        });
+
+        it("should delete again same elements in an iterable way and return current elements.", () => {
+            const result = testSet.discard([1]);
+            expect(Array.from(result)).to.eql([2, 3]);
+
+            const sameResult = testSet.discard([1]);
+            expect(Array.from(sameResult)).to.eql([2, 3]);
+        });
     });
 });