When the net/http server receives a GET request, it calls io.LimitedReader(r, 0) which
allocates an *io.LimitedReader just to read 0 bytes.

The package already contains an eofReader global variable to return 0 bytes.

Further, no need to io.Copy(ioutil.Discard, ...) from it, since it's empty, and we know
that.

This saves one allocation per GET request.

benchmark                         old ns/op    new ns/op    delta
BenchmarkServerHandlerTypeLen         13888        13279   -4.39%
BenchmarkServerHandlerNoLen           12912        12229   -5.29%
BenchmarkServerHandlerNoType          13348        12632   -5.36%
BenchmarkServerHandlerNoHeader        10911        10261   -5.96%

benchmark                        old allocs   new allocs    delta
BenchmarkServerHandlerTypeLen            20           19   -5.00%
BenchmarkServerHandlerNoLen              18           17   -5.56%
BenchmarkServerHandlerNoType             18           17   -5.56%
BenchmarkServerHandlerNoHeader           13           12   -7.69%

benchmark                         old bytes    new bytes    delta
BenchmarkServerHandlerTypeLen          1913         1878   -1.83%
BenchmarkServerHandlerNoLen            1878         1843   -1.86%
BenchmarkServerHandlerNoType           1878         1844   -1.81%
BenchmarkServerHandlerNoHeader         1085         1051   -3.13%