fix(sse): stream sink content

[thiagomini]

closes #8877

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)
• [N/A] 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 using SSE feature, the first message received by client contains only colons (":").

Issue Number: #8877

What is the new behavior?

When using SSE feature, the first message received by client contains only newline character ("\n").

Does this PR introduce a breaking change?

• Yes
• No

N/A

Other information

I chose to keep the newline character as suggested by @jpineaufr. I think it was added to pretty print the message. Also, we could improve tests readability with some utility functions to encapsulate parts of the code and make use of "give", "when" and "then", like so:

const { sse, sink } = givenANewSseWithSink()
   
whenMessageIs({
  type: 'tea-time',
  id: 'the-id',
  retry: 222,
  data: 'hello',
})

await written(sink);

expect(sink.content).to.equal(
 `
event: tea-time
id: the-id
retry: 222
data: hello

`,
    );

[coveralls]

Pull Request Test Coverage Report for Build a7ff8fd1-6101-4690-8fb1-cb33639e2040

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.13%

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Totals
Change from base Build d20b4d97-8bfd-440c-9b6c-e33520cb182a:	0.0%
Covered Lines:	5693
Relevant Lines:	6048

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💛 - Coveralls

[kamilmysliwiec]

Thanks @thiagomini, I'll review it shortly

[MarkPieszak]

LGTM 👍