If CONFIG_FEATURE_HTTPD_GZIP is enabled and request contained
'Accept-Encoding: gzip', then errors were sent with 'Content-Encoding: gzip'
even though they aren't.
Fix it by clearing content_gzip before sending the headers.
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>