Add keep_headers to the recorder to preserve named default headers

[golikovichev]

Summary

Closes #763.

The recorder strips a fixed set of verbose default headers (Content-Length, Content-Type, Date, Server, Connection, Content-Encoding) so recorded files stay terse. That is the right default, but it makes one use case impossible: a response whose signature covers the Date header cannot be verified against a recording, because the recorded file no longer has the Date value.

This adds an opt-in keep_headers argument that excludes named headers from the default stripping:

@_recorder.record(file_path="out.yaml", keep_headers=["Date"])
def test_recorder():
    ...

• Available on both record and dump_to_file.
• Matched case-insensitively, consistent with the existing default-header handling (a lowercase HTTP/2 date is kept too).
• A bare string is treated as a single header name, so keep_headers="Date" does the expected thing rather than iterating over characters.
• Defaults to None, so existing recordings are byte-for-byte unchanged.

Tests

• Unit tests on _remove_default_headers: a named header is kept while the other defaults are still stripped; a bare string is accepted; naming a non-default header is a no-op.
• An end-to-end test through record() that preserves Date with the flag and confirms it is stripped without it.

CHANGES and the README recorder section are updated.