Fix: SSE Stream parser expects additional space after colon "data:"

[michaelfeil]

This commit updates the SSE parser to handle cases where there is an optional space after the "data:" prefix. Described in detail here - #498

To address this, I modified the parser code to use the startswith method and then strip any leading or trailing whitespaces from the line. This ensures that the parser can handle both cases where there is a space after "data:" and where there isn't. The modified code now correctly decodes the line as UTF-8 and returns the parsed data.

This change improves compatibility with different SSE implementations and ensures that the parser functions correctly in various scenarios. It provides a more flexible solution that aligns with the SSE specification and accommodates libraries like springframework that omit the space after "data:".

Fixes #498

[michaelfeil]

This provides a backwards compatible and tested for #498 and proposes a slighty different solution to #502

It is according to the official standard of event stream:

the line contains a U+003A COLON character (:)
Collect the characters on the line before the first U+003A COLON character (:), and let field be that string.

Collect the characters on the line after the first U+003A COLON character (:), and let value be that string. If value starts with a U+0020 SPACE character, remove it from value.

Process the field https://html.spec.whatwg.org/multipage/server-sent-events.html#processField)

[michaelfeil]

@logankilpatrick Would you be so kind and give this a review?

[hallacy]

Awesome! Thank you for referencing the spec

[lucasgadams]

Why not use an actual SSE parsing client/library?