Allow sending empty HTTP header

[jcamiel]

With this Hurl file:

GET https://foo.com
x-foo: 
HTTP 200

We're providing the header x-foo: to libcurl. User headers like this are filtered by libcurl, they're not sent as empty headers.
This syntax is also used by libcurl to remove the HTTP request headers that are automatically added (like Host, Accept etc...).

Example: this file

GET https://foo.com
Host:
HTTP 200

will remove the Host header used by curl.

We may need to provide a way to send a user empty request header. From https://curl.se/libcurl/c/CURLOPT_HTTPHEADER.html, this is achieved by adding a header with a semi colon: x-foo;.

We could support a special syntax to indicate that we want to send an empty header

GET https://foo.com
x-foo: ""
HTTP 200

or

GET https://foo.com
x-foo: -
HTTP 200

Or, for this case, we use the incoming --header option:

GET https://foo.com
[Options]
header: x-foo;
HTTP 200

In this case, we need to make some change in the client.rs code responsible for configuring libcurl headers.

[fabricereix]

I would rather choose consistency within Hurl (standard key:value) than consistency with curl.
to be discussed...

[jcamiel]

We can change the current code so that a header name: will add an empty header (they're filtered by libcurl today). Maybe having a specific option to remove headers added by libcurl (we re just speaking about Accept and Host here) isn't worth it, given that you can override them or set it with an empty value if we change the code.