Folded header lines will now get passed through like before. The headers
API is adapted and will provide the content unfolded.
Added test 1274 and extended test 1940 to verify.
Reported-by: Petr Pisar
Fixes#8844Closes#8899
Mostly based on recent language decisions from "everything curl":
- remove contractions (isn't => is not)
- *an* HTTP (consistency)
- runtime (no hyphen)
- backend (no hyphen)
- URL is uppercase
Closes#8646
