Add support for sending empty header values

[GUI]

Currently, there's not a super-easy way to pass a header with an empty value (e.g., X-Foo: ) in Typhoeus or Ethon. If an empty string is passed in as the header value, then currently the header is omitted from the request altogether. This is due to how curl historically treated empty values (see https://curl.haxx.se/mail/lib-2010-08/0174.html). Curl 7.23.0 (released November 2011) added a more standard way to pass empty values, but it requires special syntax of using a semicolon terminator.

This changes Ethon's behavior, so that when an empty string is passed as the value of a header, it gets sent by curl as an empty HTTP header, rather than omitting the header. A nil value will still omit the header. This seems like a more intuitive approach to me, but this does make this a backwards incompatible change if you were relying on the previous empty string behavior. So if that seems problematic, let me know if you have any other ideas on how this should be handled, or if you don't think Ethon should handle this at all.

A few implementation notes:

• This requires curl 7.23.0 or higher, where the ability to send empty header values (via a semicolon terminator) was added. If using an older version of curl without this support, then curl's previous behavior of omitting empty headers when an empty string is used remains the same. While this doesn't necessarily seem ideal (since Ethon's behavior will differ depending on the version of curl you have), I'm not sure of the best alternative.
• Based on some micro-benchmarks, detecting empty strings by using the EMPTY_STRING_VALUE constant seems like the fastest approach (while .empty? is marginally faster it becomes slower once you account for checking whether or not the value is a String).
• The behavior hasn't changed if the value is any other object type, like numbers, but I added a test when passing a number just as a sanity check.

[hanshasselberg]

Hi @GUI, thank you for your work and your detailed description.

While this doesn't necessarily seem ideal (since Ethon's behavior will differ depending on the version of curl you have), I'm not sure of the best alternative.

Technically, the behavior of Ethon doesn't change, but the one of curl. Right? If thats what you meant, I don't think it is a big deal. Curls behaviour changes every once in a while. Ethon doesn't depend on a specific version.

[GUI]

Yes, you're correct. Ethon's behavior is the same with this PR: an empty string value will result in the header being sent to libcurl as semicolon terminated. How curl interprets that semicolon differs depending on the version of curl: less than version 7.23.0 will result in the header not being set (similar to the previous behavior), while 7.23.0+ will omit the header. This test demonstrates the difference: https://github.com/typhoeus/ethon/pull/132/files#diff-aa2b066a5d2286fb8a792b169fe666c1R50 Let me know if you have any other questions or suggestions.

Thanks for taking a look!