broken compilation after downgrade go-ethereum to v1.11.5

[glempi]

ethrpc/client.go have code

func createClient(ec *ethclient.Client) *Client {
	c := &Client{
		ethClient:  ec,
		gethClient: gethclient.New(ec.Client()),
	}

as u can see in this code there is calling of method 'Client()' of struct ethclient.Client
but method 'Client' was introduced only in go-ethereum to v1.12
so after downgrade go-ethereum to v1.11.5 compilation of this module was broken

[glempi]

@datluongductuan