fix(core): apply existing headers to sse responses

[wSedlacek]

PR Checklist

Please check if your PR fulfills the following requirements:

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

PR Type

What kind of change does this PR introduce?

• 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?

When sending an sse response with fastify no headers applied to the fastify reply would be applied to the headers of the sse response

Issue Number: #8717

What is the new behavior?

Any headers from the express response or fastify reply will be read and then applied to the sse response.
This has been tested with express and fastify with app.enableCors()

Does this PR introduce a breaking change?

• Yes
• No

[coveralls]

Pull Request Test Coverage Report for Build 0b4550b8-00c4-4985-80a5-b2f9b2067082

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.08%

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Totals
Change from base Build e6351c6b-4e81-4159-bd7d-395c4d0911fa:	0.0%
Covered Lines:	5658
Relevant Lines:	6014

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💛 - Coveralls

[jmcdo29]

LGTM. Thanks for adding some typings and tests around this

[wSedlacek]

@jmcdo29 What's the typical timeline on bugs like this?

[jmcdo29]

Depends on how busy the week is for Kamil. Sometimes it's the same day, sometimes it's a week or two. I'll try to point this out to him next week if he doesn't get to it over the weekend

[kamilmysliwiec]

LGTM

[maxsimych]

@kamilmysliwiec Hello, sorry for disturbing, This bug was critical for some projects, including ours, do you have ability to tag this version, to allow us to upgrade dependency from npm. Thank you anyway!