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.

# Before
if item_key.lower() == get_header_key

# After  
if item_key == get_header_key

Benchmark

# 10 headers, 100k iterations
_list = [(key, key.lower(), b'value') for key in headers]
get_header_key = b'x-header-5'

# Original
[v for _, k, v in _list if k.lower() == get_header_key]  # 0.0647s

# Fixed
[v for _, k, v in _list if k == get_header_key]          # 0.0437s

Result: 32.5% faster (1.48x speedup)