Fix static file handler for range requests on files > 2^31 bytes

[omarroth]

To reproduce on master:

require "kemal"
Kemal.run

Serving a file > ~2.1 GB, for example:

$ dd if=/dev/urandom of=file2 bs=1000 count=2147484 status=progress

Request with:

$ curl http://localhost:3000/file2 -H 'Range: bytes=0-' -v -o /dev/null

which produces:

Exception: Negative limit (ArgumentError)
  from /usr/lib/crystal/io.cr:1147:5 in 'copy'
  from /helpers.cr:218:5 in 'multipart'
  from src/kemal/helpers/helpers.cr:137:12 in 'send_file'
  from src/kemal/static_file_handler.cr:65:9 in 'call'
  from /usr/lib/crystal/http/server/handler.cr:26:7 in 'call_next'
  from src/kemal/exception_handler.cr:8:7 in 'call'
  from /usr/lib/crystal/http/server/handler.cr:26:7 in 'call_next'
  from src/kemal/log_handler.cr:8:37 in 'call'
  from /usr/lib/crystal/http/server/handler.cr:26:7 in 'call_next'
  from src/kemal/init_handler.cr:12:7 in 'call'
  from /usr/lib/crystal/http/server/request_processor.cr:39:11 in 'process'
  from /usr/lib/crystal/http/server/request_processor.cr:16:3 in 'process'
  from /usr/lib/crystal/http/server.cr:462:5 in 'handle_client'
  from /usr/lib/crystal/http/server.cr:428:13 in '->'
  from /usr/lib/crystal/fiber.cr:255:3 in 'run'
  from /usr/lib/crystal/fiber.cr:47:34 in '->'
  from ???

2019-07-31 18:36:19 UTC 500 GET /file2 112.57ms

Description

The static file handler will serve an incorrect content length when encountering a range request on files > 2^31 bytes (Int32::MAX) due to an integer overflow.

Since file.size returns a UInt64, I've updated the rest of the code to use the same type.

[straight-shoota]

It's better to use Int64 for math operation to avoid negative overflow. Int64::MAX should still be plenty of range.

[sdogruyol]

Thank you @omarroth 🙏

[asterite]

file.skip only accepts values less or equal to 1024 (buffer size, undocumented)

@sdogruyol Do you know what this means? I tried it and it works fine with values greater than1024.

[sdogruyol]

Been a long time since I touched this..seems like the statement is wrong as of now 🤔