aws_sigv4: fix canon order for headers with same prefix

If a request containing two headers that have equivalent prefixes (ex.
"x-amz-meta-test:test" and "x-amz-meta-test-two:test2") AWS

aws_sigv4: fix canon order for headers with same prefix

If a request containing two headers that have equivalent prefixes (ex.
"x-amz-meta-test:test" and "x-amz-meta-test-two:test2") AWS expects the
header with the shorter name to come first. The previous implementation
used `strcmp` on the full header. Using the example, this would result
in a comparison between the ':' and '-' chars and sort
"x-amz-meta-test-two" before "x-amz-meta-test", which produces a
different "StringToSign" than the one calculated by AWS.

Test 1976 verifies

Closes #14370

show more ...