bugfixed: nodelay of limit_req failed to work
thanks to jiyiping
fixed an problem that it may don't pass the request header

When the client_body_postpone_size is specified to be 0, it doesn't
pass the headers before receving any request body.
Don't process the encoding response in foot filter module
process the zero length response in footer module
fixed a bug of trim module.

Conflicts:

	src/http/modules/ngx_http_trim_filter_module.c
added the changelog for tengine-1.5.2
Merge pull request #347 from yaoweibin/tengine-1.5.2

Tengine 1.5.2