Ignore Range header if served file is 0 bytes

[zarqman]

Normally Rack::Files truncates a byte range to fit a file's contents--as long as at least some bytes are within range. However, an empty (0 byte) file is a special case since there are no requested bytes that can be within range.

Previously, 0 byte files requested with a Range header always resulted in a 416 error. This PR changes 0 byte files to ignore the Range header and return the empty file.

Ignoring the Range header is allowed per RFC 9110 §14.2.

This improves compatibility with clients that speculatively request files with byte ranges. For example, Nginx's slice / slice_range feature causes all requests to utilize Range.

Slightly ironically, this also results in a smaller response. (On this basis, it could even be argued to ignore Range for files below 50-100 bytes.)

Before

$ curl -i -H 'Range: bytes=0-1023' http://localhost:3000/favicon.ico
HTTP/1.1 416 Range Not Satisfiable
content-type: image/vnd.microsoft.icon
x-cascade: pass
content-range: bytes */0
Content-Length: 25

Byte range unsatisfiable

After

$ curl -i -H 'Range: bytes=0-1023' http://localhost:3000/favicon.ico
HTTP/1.1 200 OK
last-modified: Tue, 15 Jan 2019 21:51:01 GMT
content-type: image/vnd.microsoft.icon
Content-Length: 0

[ioquatix]

This seems reasonable to me, @jeremyevans @tenderlove do you have any thoughts or potential security issues?

[jeremyevans]

Seems fine to me.

[ioquatix]

This looks good to me. Can you please:

• Add an entry to the changelog.

Thanks!

[zarqman]

Changelog and comment both added.