perf: remove redundant .lower() calls in Headers

[shindonghwi]

`Headers._list` stores tuples as `(raw_key, lowercase_key, value)`.

In `get_list()` and `delitem()`, calling `.lower()` on `item_key` is redundant since it's already lowercase.

# Beforeifitem_key.lower() ==get_header_key# After ifitem_key==get_header_key

Benchmark

# 10 headers, 100k iterations_list= [(key, key.lower(), b'value') forkeyinheaders]
get_header_key=b'x-header-5'# Original
[vfor_, k, vin_listifk.lower() ==get_header_key] # 0.0647s# Fixed
[vfor_, k, vin_listifk==get_header_key] # 0.0437s

Result: 32.5% faster (1.48x speedup)