allow header and set header in client

[Stebalien]

need send Authorization header to remote exec cmds.
allow header and support set header value in client

#222 with an extra commit.

[mar1n3r0]

We could add the default allowed headers to [reforigin_test](

go-ipfs-cmds/http/reforigin_test.go

Line 53 in 40b8fdd

func originCfg(origins []string) *ServerConfig {

)

func originCfg(origins []string) *ServerConfig {
	cfg := NewServerConfig()
	cfg.SetAllowedOrigins(origins...)
	cfg.SetAllowedMethods("GET", "PUT", "POST")
        cfg.AddAllowedHeaders("Authorization", "Content-Type", "Content-Disposition") // etc any other sane defaults
	cfg.AllowGet = true
	return cfg
}