Add GoHTTPHeader to benchmarks
Add GoHTTPHeader to benchmarks

No optimization work was performed.

```
PASS
BenchmarkInject_GoHTTPHeader_Empty-8         	 1000000	      3022 ns/op	     689 B/op	      17 allocs/op
BenchmarkInject_GoHTTPHeader_100Attributes-8 	   30000	     57476 ns/op	   35573 B/op	     426 allocs/op
BenchmarkExtract_GoHTTPHeader_Empty-8        	 1000000	      1484 ns/op	     272 B/op	      12 allocs/op
BenchmarkExtract_GoHTTPHeader_100Attributes-8	   10000	    108008 ns/op	   33267 B/op	     621 allocs/op
ok  	github.com/opentracing/opentracing-go/standardtracer	8.022s
```

opentracing/basictracer-go