Assertion failed if resume called on half-closed connection

Bumped into following problem when working with half-closed client connection: resume called when socket already marked with EOF (check the test case in the PR).
With 0.8.22 the outcome is:
```
assert.js:102
  throw new assert.AssertionError({
        ^
AssertionError: false == true
    at TCP.onread (net.js:410:12)
```

I suppose that assertion could be removed. The same test works fine with current master.