Ability to not JSON.stringify simple strings

[Bo98]

For simple strings like sse.send("Test string"), you get the following output:

id: 0
data: "Test string"

However it is perfectly valid to have, and perhaps more common to have:

id: 0
data: Test string

If this representation is desired however, it is not possible to get it currently using express-sse.

[ekoeryanto]

I think that's simple to do.

const stringify = (val) => typeof val === 'string' ? val : JSON.stringify(val)

Ah, just do it with PR