gzipResponseWriter needs to be a pointer for the sniffDone optimization

fixes #1. as a value type the sniffDone value was lost between Write
calls. the sniffDone bool helps avoid a map lookup for Content-Type
between multiple write calls. this changes the behaviour as now we only
sniff in the first write call rather than every write call, which is
probably the right thing to do anyways. may want to cache some fixed
size of bytes to do a more thorough detect though.