Abort works incorrectly for delayed requests

**I'm submitting a bug report**

* **Library Version:**
1.0.4

**Please tell us about your environment:**
* **Operating System:**
OSX 10.x

* **Browser:**
Chrome 56 | Firefox 51

* **Language:**
es2015

**Current behavior:**

An interceptor can delay XHR sending, and if the app cancels the request before it was send then the promise never resolved. HttpClient.pendingRequests collection grows with each request. Sample code:

```
initTest() {
console.log('Test')
this.testClient = new HttpClient();
this.testClient.configure(config => {
config.withInterceptor({
request(message) {
//Delay request for 1 second
return new Promise((resolve, reject) => {
setTimeout(() => {resolve(message)}, 1000)
})
},
});
});
}

test() {
console.log('Test')
let promise = this.testClient.get('/test'); //preserve original promise with abort method
promise.then(response => {
console.log('response', response)
})
.catch(e => {
console.log('error', e)
})
promise.abort()
console.log('Pending requests', this.testClient.pendingRequests)
}

```
https://github.com/aurelia/http-client/blob/master/src/request-message-processor.js line 152 is invoked during processing phase.

```
if (this.isAborted) {
// Some interceptors can delay sending of XHR, so when abort is called
// before XHR is actually sent we abort() instead send()
this.xhr.abort();
} else {

```
Line 140 - `xhr.onabort = (e) => { `never invoked

**Expected/desired behavior:**
Expected result - promise either resolved or rejected

Start in src/request-message-processor.js around lines 140 and 152, then reproduce the delayed-interceptor example in Chrome or Firefox. Trace the abort path when XHR has not been sent yet; done means the original promise resolves or rejects and HttpClient.pendingRequests does not retain the cancelled request.

Written by the indexing model from the issue text.