Please let HttpParams.set and .append accept one object-like arg with nested data

## I'm submitting a...
[ x ] Feature request

## Current behavior

You can only set or append to HttpParams one by one.

## Expected behavior

With Http.get, .post, etc, you could pass any object with nested object-like data as options.params. First level keys would be mapped to query params, and values JSON encoded if they're object-like. I'd like to achieve the same without adding complexity by doing something like:

> new HttpParams.set( { where: { id: [ 1,2,3 ] } } )

## What is the motivation / use case for changing the behavior?

In migrating to HttpClient from Http, I had to add a helper method:
```
private anyToHttpParams(obj: any) { // requires lodash
return Object.entries(obj)
.reduce(
(params, [key, value]) => params.set(
key, _.isObjectLike(value) ? JSON.stringify(value) : value
),
new HttpParams()
);
}

// this.http.get(url, { params: this.anyToHttpParams({ where: { id: [ 1,2,3 ] } }) } );
```

For me, this was sort of a breaking change as well, as I didn't get a compiler error when I just replaced Http with HttpClient and passed objects with nested data to params like I used to with Http, but in network console my xhr URLs changed to something like `/action?[object object]`.

## Environment

Angular version: 5.0.1

Start with the HttpParams.set and append APIs and the HttpClient options.params path described in the issue. Done means one object-like argument maps first-level keys to query parameters and JSON-encodes object-like values, including nested data.

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