Pkg: httpHeaderNormalizer to handle canonical sanitized headers

[ScoobySmack2612]

Given canonical: true,
And an event containing a header that's "sanitized" (X----)
A runtime error occurs, "Cannot read property 'toUpperCase' of undefined".

To Reproduce:

Unit test

test('It should not fail given a corrupted header key', async (t) => {
  const handler = middy((event, context) => event)

  handler.use(httpHeaderNormalizer({ canonical: true }))

  const event = {
    headers: {
      'X----': 'foo'
    }
  }

  const expectedHeaders = {
    'X----': 'foo'
  }

  const originalHeaders = { ...event.headers }

  const resultingEvent = await handler(event, context)

  t.deepEqual(resultingEvent.headers, expectedHeaders)
  t.deepEqual(resultingEvent.rawHeaders, originalHeaders)
})

Thrown error: "Cannot read property 'toUpperCase' of undefined".

Expected behavior: Should simply return the raw header

Additional context:
We had a client who appears to be sanitizing headers resulting in our lambdas receiving a header that looks like X----. Though this is a one-off, rather than push the onus to our client, an empty string fallback gets them back up an running.

[willfarrell]

I think this was added to the wrong file. Can you do a quick update please.

[ScoobySmack2612]

Thank you, sorry about that.

[willfarrell]

X---- header? Can say I've never seen that before. I would still tell the client to update X---- to something more standard. Thanks for the PR and quick update.

[ScoobySmack2612]

X---- header? Can say I've never seen that before. I would still tell the client to update X---- to something more standard. Thanks for the PR and quick update.

Its definitely not, I suspect they have just enough of a proxy configuration to get themselves in trouble. I suspect they're stripping unknown headers, or something similar. Thanks for a speedy review!