Feature/send headers when using node event source transport

[Qantt]

Description

I am using this package to communicate with SSE MCP server. I noticed that this package was not sending headers in node environment. This PR addresses this by starting to send headers there as well.

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

How Has This Been Tested?

• Tests were updated to verify inclusion of the headers.
• This has been manually tested by using this to communicate (and in this case to send headers) to an SSE MCP server.

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes