tests: fixed a test case that could fail randomly.
bugfix: ngx.req.socket(true): it incrrectly returned the error "chunked request bodies not supported yet" for *raw* request sockets with chunked request bodies. thanks Xiaofei Yang for the report.
bugfix: ngx.resp.get_headers(): sometimes we might omit the builtin-headers Content-Type, Content-Length, Connection, and Transfer-Encoding. thanks Jon Keys for the report.
tests: made a test case for ngx.on_abort() more tolerant.