Write overflow - test validating truncated writes

[simonferquel]

This adds a test (that fails on current master), ensuring writes are correctly truncated by the channel, according to msize

[simonferquel]

@stevvooe this test should make it easier to validate #30.
I am not completely happy with the fake net.Addr / net.Conn implementation (it is quite verbose), but it correctly exhibits the problem.

I'll do a second PR to test for read overflows.

[stevvooe]

I am not completely happy with the fake net.Addr / net.Conn implementation (it is quite verbose), but it correctly exhibits the problem.

When mocking an interface, it is easiest to start with a stub, embedded implementation and then implement the calls required for the tests:

type mockConn struct {
  net.Conn
}

The above implements net.Conn with a nil receiver. Any calls to unimplemented methods will panic. When you want behavior, simply override the target method:

func (c *mockConn) Write(b []byte) (n int, err error) {
  c.data = append(c.data, b...)
  n = len(b)
  return n, nil
}

This looks okay. I can make some adjustments in #30 after a few fixes.

[stevvooe]

Make sure to check the error here.

[stevvooe]

@simonferquel I took on this test and few others in #30. Closing.