added TENGINE macro as "Tengine"
adde a member 'server_tag_header' in ngx_http_core_loc_conf_t to replace 'server_tag'
now the 'server_tokens' and 'server_tag' directives can change the server banner in a default error page.
added test cases
reverted version to 1.5.0
Merge pull request #264 from zhuzhaoyuan/suppress-server-banner

Suppress server banner in error pages