fix coverage issue

Author: loreanvictor

Diff for: src/test/select.test.ts

@@ -4,6 +4,21 @@ import { select } from '../select'
 
 
 describe(select, () => {
+  test('proxies a source.', () => {
+    const src = new State(0)
+    const selected = select(src)
+
+    const cb = jest.fn()
+
+    observe($ => cb($(selected)!))
+
+    src.set(1)
+    expect(cb).toHaveBeenCalledWith(1)
+
+    src.set(2)
+    expect(cb).toHaveBeenCalledWith(2)
+  })
+
   test('without select, higher order sources are stopped after switch.', () => {
     const flag = new State(false)
     const a = new State('a')