Discard binary HTTP response bodies by default

[na--]

Unless users have explicitly specified that they want to keep the body of a particular http.batch() request (i.e. have set its responseType to binary or text) that returns a binary content-type, we probably should discard its response body. The actual body contents of things like images, videos and other binary files are very rarely actually needed, and discarding them would save a ton of memory when discardResponseBodies isn't turned on.

Some things to consider:

• Non-batch() HTTP requests should probably also be treated the same way, both for consistency and to reduce the implementation complexity
• We probably need to introduce a new property in the Response object for the response body's length, since we currently would loose it, and it may be a useful property for checks.
• Whether we should use a whitelist or a blacklist for the content types we want to discard... I think by default we should keep the body, unless the content-type matches a very restrictive blacklist like ^image/.+|audio/.+|video/.+)$ (and maybe include some or all of the application/ MIME types)
• That list should probably be user-configurable, at least via the exported options...

[tom-miseur]

A check against the Content-Type response header is usually all one needs to determine success of a request resulting in binary content, although being able to also check the response length might be useful, but much less critical. I don't think I've ever seen a server respond with the expected Content-Type when it isn't actually the "thing" you're expecting; what tends to happen is you get an error page which is then Content-Type: text/html (or something else).
Given the memory improvements this would yield, I'd say it's worth implementing as a sensible default.