[internal] nil pointer when fetch system info

[at15]

/Users/piguo/app/go/bin/go run /Users/piguo/workspace/src/github.com/at15/go-solr/example/job/main.go
example for storing job log in solr
panic: runtime error: invalid memory address or nil pointer dereference
creating solr client
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x11e3d84]

goroutine 1 [running]:
net/http.(*Request).WithContext(0x0, 0x139a860, 0xc420010228, 0xc4200c0600)
	/Users/piguo/app/go/src/net/http/request.go:332 +0x44
github.com/at15/go-solr/pkg/internal.(*Client).Do(0xc4200146e0, 0x139a860, 0xc420010228, 0x0, 0x1225080, 0xc4200cc580, 0x0, 0x0, 0x0)
	/Users/piguo/workspace/src/github.com/at15/go-solr/pkg/internal/client.go:74 +0x7e
github.com/at15/go-solr/pkg/internal.(*Client).Get(0xc4200146e0, 0x139a860, 0xc420010228, 0x1293415, 0x17, 0x1225080, 0xc4200cc580, 0xc42000e038, 0xc420049eb8, 0x12078e4)
	/Users/piguo/workspace/src/github.com/at15/go-solr/pkg/internal/client.go:70 +0xe8
github.com/at15/go-solr/pkg/admin.(*Service).SystemInfo(0xc42000e038, 0x139a860, 0xc420010228, 0xc42000e018, 0xc42000e038, 0xc4200146e0)
	/Users/piguo/workspace/src/github.com/at15/go-solr/pkg/admin/service.go:20 +0x86
github.com/at15/go-solr/pkg.(*SolrClient).IsUp(0xc420016480, 0x139a860, 0xc420010228, 0xc420016480, 0x0)
	/Users/piguo/workspace/src/github.com/at15/go-solr/pkg/client.go:65 +0x47
main.main()
	/Users/piguo/workspace/src/github.com/at15/go-solr/example/job/main.go:19 +0x1e7
exit status 2