🐞 bug report

Affected Package

The issue is caused by package @angular/common/http

Is this a regression?

I just started using the package so I'm not sure if it worked for previous versions

Description

Passing an instance of browser-native UrlSearchParams as the second argument to HttpClient#post (or as the body option for any request with a body) causes an empty JSON object string "{}" to be used as the body, instead of the correctly serialized URL-encoded form string.

🔬 Minimal Reproduction

https://angular-issue-repro2-cs1zmj.stackblitz.io

🌍 Your Environment

Angular Version:

Angular CLI: 9.0.3
Node: 12.7.0
OS: win32 x64

Angular: 9.0.2
... common, compiler, core
Ivy Workspace: 

Package                      Version
------------------------------------------------------
@angular-devkit/architect    0.900.3
@angular-devkit/core         9.0.3
@angular-devkit/schematics   9.0.3
@angular/cli                 9.0.3
@schematics/angular          9.0.3
@schematics/update           0.900.3
rxjs                         6.5.3

Anything else relevant?
I've tested this in latest Edge, Firefox, and Chrome. (IE doesn't support native UrlSearchParams.)