Azure/fetch-event-source

📋 Changed

• The `id`, `event`, and `data` fields are now initialized to empty strings, per the spec (they were previously `undefined`)
• The `onmessage` callback is now called for _all_ messages (it was previously triggered only for messages with a `data` field)
• If a message contains multiple `data` fields, they will be concatenated together into a single string. For example, the following message:
• ````
• data: Foo
• data:Bar
• data
• data: Baz
• + 3 more

🗑️ Removed

• The internal `parseStream` function has been removed. The parse implementation was previously based on async generators, which required a lot of supporting code in both the typescript-generated polyfill and the javascript engine. The new implementation is based on simple callbacks, which should be much faster.

📋 Changed

• Updated examples in readme to fix typos, added more comments.
• Changed `if` statements in parse.ts to test for specific values instead of truthy/falsy values.

📋 Changed

• Changed the default onOpen validator to allow charset and boundary directives in the content-type (Fixes #1)