httpd: When sending gzipped content use content-length header

httpd: When sending gzipped content use content-length header

Today for gzipped content httpd is using a header with name
Transfer-Length. However I can't find a header with that name in the
standards. Instead use Content-Length.

function                                             old     new   delta
.rodata                                           157940  157936      -4
send_headers                                         980     939     -41
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-45)             Total: -45 bytes

Signed-off-by: Alexander Vickberg <wickbergster@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>