Add Go 1.12 support

accept.go

accept_test.go

autobahn_test.go

close.go

conn.go

conn_test.go

dial.go

dial_test.go

example_echo_test.go

go.mod

internal/errd/wrap.go

netconn.go

read.go

write.go

ws_js.go

wsjson/wsjson.go

wspb/wspb.go