Support setHeader for more compatibility with Express middleware

[AndrewBarba]

Currently we can set headers using res.header(key, value) however a lot of Express middleware, ex. Helmet, use res.setHeader(key, value).

Adding a setHeader alias would allow these types of middleware to work out of the box.

I currently add res.setHeader = (...args) => res.header(...args) to all of my projects that use lambda-api in order to support this

[jeremydaly]

Makes sense.