Fix start() re-entrancy (#1)

[g-mark]

Fixes an EventSource.start() re-entrancy problem

When:

1. EventSource.start() is called
2. EventSource.start() is called again, before the network response from the first call has been received

Then:

• a second URLSession and URLRequest are created, and a new URLSessionDataTask is started

The underlying issue is that start() is checking for readyState == .raw, but readyState isn't set until a network response has been received.

Requirements

• [x] I have added test coverage for new or changed functionality
• [x] I have followed the repository's pull request submission guidelines
• [ ] I have validated my changes against all supported platform versions

Related issues

None

Describe the solution you've provided

Set readyState = .connecting in start(), immediately after checking for readyState == .raw.

Describe alternatives you've considered

None

Additional context

None

[tanderson-ld]

Hi there @g-mark , thank you for the contribution! I have it on my list to take a look at the changes and related code/behavior and if it looks good, we'll integrate the change.