- Handle user headers in format 'name:' and 'name;' with no value.
The former is used when the user wants to remove an internal libcurl
header and the latter is used when the user actually wants to send a
no-value header in the format 'name:' (note the semi-colon is converted
by libcurl to a colon).
Prior to this change the AWS header import code did not special case
either of those and the generated AWS SignedHeaders would be incorrect.
Reported-by: apparentorder@users.noreply.github.com
Ref: https://curl.se/docs/manpage.html#-H
Fixes https://github.com/curl/curl/issues/11664
Closes https://github.com/curl/curl/pull/11668
