Problem when sending request with empty header

[jdecool]

Guzzle version(s) affected: 7.x.
PHP version: 7.2.x
cURL version: 7.52.1

Description
Two years ago an issue was opened because of a problem with sending empty headers.

This issue was fixed by this PR.

The fix worked with headers defined manually, but for headers which are set automatically by Guzzle, the fix was not applied.

Examples:

• https://github.com/guzzle/guzzle/blob/master/src/Handler/CurlFactory.php#L329
• https://github.com/guzzle/guzzle/blob/master/src/Handler/CurlFactory.php#L390
• https://github.com/guzzle/guzzle/blob/master/src/Handler/CurlFactory.php#L303
• https://github.com/guzzle/guzzle/blob/master/src/Handler/CurlFactory.php#L308
• and some others

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed after 2 weeks if no further activity occurs. Thank you for your contributions.