Fails to pick up changes from gulp-watch

I'm unable to make gulp-compass compile anything coming piped through gulp-watch.

This works:

```
return gulp.src('app/styles/base.scss')
.pipe(compass({
sass: 'app',
css: '.tmp'
}));
```

This doesn't work:

```
return gulp.src('app/styles/base.scss')
.pipe(watch('app/styles/base.scss'))
.pipe(compass({
sass: 'app',
css: '.tmp'
}));
```

The pipe does produce a vinyl on change (I verified with gulp-debug), but gulp-compass fails to compile them, as soon as I add the watch pipe.

Start by comparing the two pipeline examples and tracing how the gulp-watch-produced vinyl reaches gulp-compass; the issue reports that gulp-debug sees the vinyl but compilation fails. Reproduce both cases, then verify that changes from the watched pipeline compile successfully and produce the expected CSS output.

Written by the indexing model from the issue text.