send multipart/form-data insteadof x-www-form-urlencoded.

I’m submitting a…

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ x ] Bug report  
[ ] Performance issue
[ ] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question
[ ] Other... Please describe:

Current behavior

    let params = new FormData();
    for (let key in options) {
      params.append(key, options[key]);
    }

    const headers = new HttpHeaders();
    headers.set(
      "Content-Type",
      "application/x-www-form-urlencoded;"
    );
    return this.http.post(url, params, { headers });

look to browser to find the request param:

Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryEjmSsnYOjN4jCZRK

Expected behavior

the request content-type should be

Content-Type: application/x-www-form-urlencoded;

Minimal reproduction of the problem with instructions

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

Environment

Angular version: X.Y.Z

```
ng --version

     _                      _                 ____ _     ___
    / \   _ __   __ _ _   _| | __ _ _ __     / ___| |   |_ _|
   / △ \ | '_ \ / _` | | | | |/ _` | '__|   | |   | |    | |
  / ___ \| | | | (_| | |_| | | (_| | |      | |___| |___ | |
 /_/   \_\_| |_|\__, |\__,_|_|\__,_|_|       \____|_____|___|
                |___/
    

Angular CLI: 6.2.4
Node: 10.10.0
OS: linux x64
Angular: 6.1.9
... animations, common, compiler, compiler-cli, core, forms
... http, language-service, platform-browser
... platform-browser-dynamic, router

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.8.4
@angular-devkit/build-angular     0.8.4
@angular-devkit/build-optimizer   0.8.4
@angular-devkit/build-webpack     0.8.4
@angular-devkit/core              0.8.4
@angular-devkit/schematics        0.8.4
@angular/cli                      6.2.4
@ngtools/webpack                  6.2.4
@schematics/angular               0.8.4
@schematics/update                0.8.4
rxjs                              6.2.2
typescript                        2.9.2
webpack                           4.20.2
```

Browser:
- [ x ] Chrome (desktop) version XX
- [ ] Chrome (Android) version XX
- [ ] Chrome (iOS) version XX
- [ ] Firefox version XX
- [ ] Safari (desktop) version XX
- [ ] Safari (iOS) version XX
- [ ] IE version XX
- [ ] Edge version XX
 
For Tooling issues:
- Node version: XX  
- Platform:  

Others:

[calidion]

@alxhub

Do you have any test case to prove that you are right before you close this issue?

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