fix: handling of array values for AxiosHeaders (#5085)

Co-authored-by: Jay <jasonsaayman@gmail.com>

test/unit/core/AxiosHeaders.js

@@ -327,5 +327,15 @@ describe('AxiosHeaders', function () {
         bar: '3'
       });
     });
+
+    it('should support array values', function () {
+      const headers = new AxiosHeaders({
+        foo: [1,2,3]
+      });
+
+      assert.deepStrictEqual({...headers.normalize().toJSON()}, {
+        foo: ['1','2','3']
+      });
+    });
   });
 });