proxyReq behavior change? Now only runs for values matched in `proxies` config.

### Expected behaviour
If a `proxyReq` function is set, it should run for all requests, not just requests that match paths defined in `proxies` config object.

### Actual behaviour
This worked as expected (`proxyReq` always ran, if defined) prior to upgrading to Angular 6 / Angular CLI 6. Now, `proxyReq` only seems to run if one or more paths are set in `proxies`, and only runs for paths that match those defined in `proxies`. I'm not sure when this changed (2.0?), or if it is actually related to the version of `node-http-proxy` being used.
(Edit: I'm on 2.0.2, so I don't think this change could be the culprit, but just in case: https://github.com/karma-runner/karma/commit/0dc8ea4ab5d281e62d324ab02bc79acd1bdd38b1#diff-a043d5a6a7b3405edd843f08f24e3da1)

### Environment Details

- Karma version (output of `karma --version`): 2.0.2
- Relevant part of your `karma.config.js` file

```
proxyReq: function( clientRequest, incomingMessage, serverResponse, options ) {
console.log( "Karma proxyReq function called." );
},
```

### Steps to reproduce the behaviour

1. Set up a `proxyReq` function in the Karma config, but define no `proxies`.
2. Run Karma and note that `proxyReq` function never runs
3. Add a path to `proxies` that will **not** be used during your Karma run
4. Run Karma, and note that `proxyReq` is still not called
5. Add a path to `proxies` that **will** be used during your Karma run
6. Run Karma, and note that `proxyReq` is now called

### Justification

Our app conditionally uses a set of mock JSON files to simulate a back-end server (based on runtime configuration). We find this very useful, for many reasons:
- App can be run without any back-end server
- App can be run with a back-end server, but endpoints that are not ready can be stubbed with a mock JSON file, so UI work can continue while server work is in progress
- Tests can also use the mock JSON data. This avoids having to painstakingly add manual mock server results to all tests, and ensures that the tests use exactly the same data that users see if they run the app without a server.

All that leads to a simple issue: many app server calls are not GET requests, and non-GET requests will fail when the target is a static JSON file. We use the `proxyReq` to change the request method from POST/PUT/DELETE to GET, so the calls can proceed without issue.

Once I figured out that a `proxies` entry was required, I tried simply doing this to get the `proxyReq` to run:

```
proxies: {
'/assets/mockdata/': '/assets/mockdata/'
},
```

But this just triggers an infinite loop in the handling of the request. The only alternative is to use a placeholder path for the mock JSON URLs, and use `proxyReq` to rewrite the path in addition to changing the request method.

Ideally, `proxyReq` would run whether any `proxies` are defined or not. Or, in lieu of that, provide some way for a path in `proxies` to trigger `proxyReq`, while indicating that the target for the proxy path should just remain the same as the original path and without triggering an infinite loop.