Add NotContentTypes option

[jared2501]

Use case: I would like to gzip all content-types, except images/videos.

[jared2501]

hey @fsouza, @jprobinson - would ya'll be interested in this change? If so, happy to add tests / refactor.

[jprobinson]

@jared2501 this seems like a reasonable addition to me. The code looks like it's in a good place. Go ahead and add some tests and I'll bring this in. Thanks!

[fsouza]

oh sorry I missed this. Yeah I second @jp's comment and I was actually thinking about this over the weekend (we have a gizmo service that generates video thumbnails and we don't want to gzip jpg images :D).

[adammck]

Hi! Because I like functional options so much, I might personally implement this api as:

interface contentTypeMatcher {
  Matches(contentType string) bool
}

func ContentTypeMatchers(matchers ... contentTypeMatcher) option {
   // ...
}

gzip.GzipHandlerWithOpts(gzip.ContentTypeMatchers(
  gzip.MatchLiteral("text/plain"),
  gzip.MatchLiteral("image/jpg", invert=True), // does go have named params? i forgot
))

...so that in some magical future, you might do such weird things as:

gzip.MatchPrefix("text/")
gzip.MatchRegex("*/*+json")
gzip.MatchFunc(func(ct string) bool {
  return time.Now().Hour < 12
})

but then I'd probably add a helper that looks just like what you're proposing, so I dunno.

[mkraft]

I have opened a PR for this also: #98