Improved event stream checking example

[slavarazum]

A tiny example update replacing equal condition for EventStreamContentType by checking with startsWith function.

- response.headers.get('content-type') === EventStreamContentType
+ response.headers.get('content-type').startsWith('text/event-stream')

covers cases when charset present: Content-Type: text/event-stream; UTF-8.

Added import of EventStreamContentType variable to take it more clear.