fix when undefined is provided

Author: maciej-ka

src/core/instance/inject.js

@@ -57,7 +57,7 @@ export function resolveInject (inject: any, vm: Component): ?Object {
         }
         source = source.$parent
       }
-      process.env.NODE_ENV !== 'production' && typeof result[key] === 'undefined' &&
+      process.env.NODE_ENV !== 'production' && Object.keys(result).indexOf(key) < 0 &&
         warn(`Injection "${key}" not found`, vm)
     }
     return result

test/unit/features/options/inject.spec.js

@@ -256,12 +256,13 @@ describe('Options provide/inject', () => {
     const vm = new Vue({
       provide: {
         foo: 1,
-        bar: false
+        bar: false,
+        baz: undefined
       }
     })
     new Vue({
       parent: vm,
-      inject: ['foo', 'bar'],
+      inject: ['foo', 'bar', 'baz'],
       created () {}
     })
     expect(`Injection "foo" not found`).not.toHaveBeenWarned()