consider second(and furthermore) calls to writeWrapper's WriteHeader() valid

gzip middleware is now beta

writerWrapper: simplify Write()

• writerWrapper: buffer writes to decide whether use gzip or not
• writerWrapper: detect Content-Type if there's none
• ginGzipWriter: full implementation for gin.ResponseWriter excluding Pusher()

fix: OPTIONS request should not be processed by gzip

fixes panic on second calling to Write()

handler: handle Gin 404

reuses writerWrapper to gain less allocations and GC pressure.

use Aho-Corasick for string matching

working draft