fix(http): preserve header case when copying headers

[alxhub]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

#12106 attempted to preserve the case of HTTP headers added to the Headers object.

It failed, because StaticRequest copies the headers (

angular/modules/@angular/http/src/static_request.ts

Line 97 in 69f006c

this.headers = new Headers(requestOptions.headers);

) and that copy ignores the saved case (

angular/modules/@angular/http/src/headers.ts

Line 52 in 69f006c

headers._headers.forEach((values: string[], name: string) => {

).

What is the new behavior?

Copying headers preserves their case.

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[x] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

[vicb]

I think the fix should be

    if (headers instanceof Headers) {
      headers._headers.forEach((values: string[], name: string) => {
        values.forEach(value => this.append(name, value));
      });
      return;
    }

->

    if (headers instanceof Headers) {
      headers.forEach((values: string[], name: string) => {
        values.forEach(value => this.append(name, value));
      });
      return;
    }

(removing ._headers)

[alxhub]

@vicb updated with your suggested simplification

[vicb]

expect(JSON.stringify(new Headers(headers))).toEqual('{"fOo":["bat"]}');

[alxhub]

Done.

[ModalSeoul]

Can't wait for this to get pulled.

[ModalSeoul]

This issue persists.

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}