Add something like part.get() or part.header()

[borekb]

It would be good to have something like part.get(name: string) (express-style) or part.header(name: string) (more logical, IMO) to be able to get HTTP headers using case-insensitive search. Currently, the only way to get the headers is from the part.headers array which strictly uses lower-case strings, i.e., not necessarily what came from the client.

[dougwilson]

PRs welcome!