Remove key_file and cert_file from kwargs since HTTPConnection isn't expecting them.
Encode request body as base64 if it contains binary data.
Store response body as simple text if possible.

This doesn't break compatibility with existing traces, since it doesn't
affect the request key, and existing responses will be decoded from
body_quoted_printable if that's how they were initially recorded.
Strip newline pairs from quopri encoded response; they occur randomly and make searching impossible.
Repair logstr to avoid double slash, since url starts with slash.