This patch makes sure that we now handle correctly bytes and native
strings on python 3:
- In python 3, sockets are now taking and returning bytes.
- according to PEP3333, headers should be native strings and body in
bytes.
Intoduce a change in log access format:
- request headers are now added to the log format using the
{HeaderName}i variable
- response headers are now handled using the {HeaderName}o variables
- headers name are insensitive
- non existent keys in the log access format are replaced by '-'