fix cohttp not working with HTTP line folding

[husong998]

Hi cohttp devs! I'm submitting this patch to fix #1033.

I know that it's best to write a regression test, but I'm not sure how to compile the codebase, letting alone adding a test case. Here's a response that can reproduce the error:

Content-Type: application/json
Content-Disposition: inline;filename=f.txt
Expect-CT: max-age=3600
X-Frame-Options: DENY
Strict-Transport-Security: max-age=31536000; includeSubDomains
X-Frame-Options: SAMEORIGIN
X-XSS-Protection: 1; mode=block
X-Forwarded-Proto: https
X-Content-Type: nosniff
Strict-Transport-Security: max-age=31536000; includeSubdomains; preload
Content-Security-Policy: default-src 'self'; script-src 'self'; style-src 'self'; img-src 'self'; font-src 'self'; connect-src 'self'; frame-src 'self';
            object-src 'self'; base-uri 'self'; form-action 'self'; frame-ancestors 'self'; upgrade-insecure-requests

Please help me get this PR merged, thanks!