Check Content-Length against minSize and passthrough writes if no gzip

[jameshartig]

Currently there's no way to generate a HEAD response with the correct
headers as the GET unless you set the minSize as 0, but then gzip headers
will be written in Close. Instead, allow a Write(nil) that will set the
correct headers based on the Content-Length/Content-Type headers and only
initialize a writer if there is a non-zero-length Write. If the
Content-Length cannot be determined, you cannot generate the response
because it cannot know if minSize would've been met.

Fixes #70

[jameshartig]

I also handled the issue mentioned in #64.

The benchmarks seem to show the BenchmarkGzipHandler_S2k-8 got faster while the BenchmarkGzipHandler_P20k-8 and BenchmarkGzipHandler_P100k-8 got slower.

Before:
BenchmarkGzipHandler_S2k-8     	     200	   8180495 ns/op
BenchmarkGzipHandler_S20k-8    	     100	  24256560 ns/op
BenchmarkGzipHandler_S100k-8   	      20	 100509175 ns/op
BenchmarkGzipHandler_P2k-8     	    1000	   1766909 ns/op
BenchmarkGzipHandler_P20k-8    	     200	   5878452 ns/op
BenchmarkGzipHandler_P100k-8   	      50	  26159506 ns/op

After:
BenchmarkGzipHandler_S2k-8     	     200	   6932798 ns/op
BenchmarkGzipHandler_S20k-8    	     100	  24246411 ns/op
BenchmarkGzipHandler_S100k-8   	      20	 101508020 ns/op
BenchmarkGzipHandler_P2k-8     	    1000	   1691251 ns/op
BenchmarkGzipHandler_P20k-8    	     200	   7326576 ns/op
BenchmarkGzipHandler_P100k-8   	      30	  33444100 ns/op

[jameshartig]

@jprobinson Not sure if I should @ you or if you get notifications from PRs. Thanks!

[jprobinson]

@fastest963 Hi! Sorry, I've been a bit busy. I've seen this but will need to find some time to review the PR. I'll try and get to it soon.

[jameshartig]

@jprobinson okay, thanks! No problem, just wanted to check on it!

[jprobinson]

🤐 🌮 !

This looks pretty good! Sorry it took me so long to review it.

[jprobinson]

thanks again, @fastest963!

[jameshartig]

No problem! Thanks for merging!