aiohttp: Match response header names case-insensitively.

[pablogventura]

Summary

Fixes #1126. Response header field names are case-insensitive (RFC 9110
Sec. 5.1), but ClientSession._request matched Transfer-Encoding and
Location with fixed capitalization via startswith. Servers that send
transfer-encoding: or location: would skip chunked detection or
redirects.

Match those names with line.lower().startswith(...), same idea as the
sketch on the issue and prior art in #523 (for old urequests, superseded
after the rename to requests). The Location value is still taken from
the original line so URL case is preserved.

Testing

• mpy-cross on aiohttp/__init__.py (compiled as __init__.py):
  • master: 3787 bytes
  • this PR: 3796 bytes (+9)
• No automated aiohttp package test in CI yet; change is two comparisons in
  the header parse loop.

Trade-offs and Alternatives

• aiohttp-only. The same wire-parse pattern exists in requests and can be
  a small follow-up; left out here to keep the diff minimal and avoid
  overlapping the open chunked-response work.
• Outgoing request header dict case-insensitivity (e.g. a _Headers
  helper) is a larger change and out of scope for this issue.

Generative AI

I used generative AI tools when creating this PR, but a human has checked the
code and is responsible for the code and the description above.

[dpgeorge]

see related #523

[pablogventura]

Thanks. #523 looks like the same wire-parse issue in requests
(Transfer-Encoding / Location matched case-sensitively).

Would you rather I fold that fix into this PR, or keep a separate
small PR for requests? (If separate, I'd wait until #1135 merges so
it does not collide with the chunked work on the same header loop.)

[dpgeorge]

I think we can just merge #523 (after resolving merge conflicts). And then this PR is independent.

But, would be good to optimize this one to only call .lower() once, per #523.

[pablogventura]

Thanks. Applied in 8678eea: one lowerl = line.lower() per header line, same pattern as #523.

__init__.mpy (mpy-cross, compiled as __init__.py):

Version	bytes	vs master
master	3787	-
two .lower() calls	3796	+9
8678eea (lowerl once)	3803	+16

I'll put the requests side in a separate PR (current requests, post-#1135) rather than rebasing #523, since that PR still targets the old urequests path.

[dpgeorge]

Thanks, looks good.