Some headers keys in 2.8.1 are in lowercase

Header names: some in lowercase

We just upgraded our play apps from 2.6.6 to 2.8.1
On our test environments we saw 502 errors coming back from other servers we call via play ws.
It seems that the newer version some headers are now in lower case.

Result from test with 2.8.1:
POST / HTTP/1.1 Content-Type: text/xml;charset=UTF-8 content-length: 340 host: localhost:9001 accept: */* user-agent: AHC/2.1

result from 2.6.6:

POST / HTTP/1.1 Content-Type: text/xml;charset=UTF-8 Content-Length: 340 Host: localhost:9001 Accept: */* User-Agent: AHC/2.0

Our endpoint broke on the content-length (lowercase) header key.
We created a workaround by explicitly setting the header with the key in the right format ("Content-Length"), but as some keys are in lower keys, and I think the RFC mentions them with first letter in uppercase, this might be a bug.

Play WS Version (migrated from 2.6.6 to 2.8.1

We migrated from 2.6.6 to 2.8.1 and encountered above issue.

API (Scala / Java / Neither / Both)

Scala

Reproduce the two request examples against Play WS 2.8.1 and 2.6.6, then trace the header-writing path used by the HTTP client. Confirm the expected behavior for the reported Content-Length case and add a regression test that distinguishes the versions.