Fix: Passing in a channel via the request map

This fixes issue `#31` - see [console error when passing a channel](https://github.com/r0man/cljs-http/issues/31).
`(<! (client request)` will return `nil` when the respective channel closes during the `EventType.COMPLETE` handler - see ([`core.clj`](https://github.com/yalh/cljs-http/blob/fix-prepared-channel/src/cljs_http/core.cljs#L42)).
Those `nil` values shouldn't be propagated, obviously. Hence the `if-let`.