Fixes current go vet warnings

This was the list of errors output from `go vet`:

```
$ go vet
channel.go:339: unreachable code
channel.go:368: unreachable code
channel.go:398: unreachable code
channel.go:784: unreachable code
channel.go:828: unreachable code
client_test.go:129: unreachable code
client_test.go:185: arg c for printf verb %s of wrong type: *amqp.Connection
client_test.go:212: arg c for printf verb %s of wrong type: *amqp.Connection
client_test.go:232: arg c for printf verb %s of wrong type: *amqp.Connection
client_test.go:248: arg c for printf verb %s of wrong type: *amqp.Connection
client_test.go:253: arg ch for printf verb %s of wrong type: *amqp.Channel
client_test.go:346: arg c for printf verb %s of wrong type: *amqp.Connection
client_test.go:351: arg ch for printf verb %s of wrong type: *amqp.Channel
client_test.go:398: arg c for printf verb %s of wrong type: *amqp.Connection
client_test.go:403: arg ch for printf verb %s of wrong type: *amqp.Channel
client_test.go:414: arg c for printf verb %s of wrong type: *amqp.Connection
client_test.go:431: arg c for printf verb %s of wrong type: *amqp.Connection
client_test.go:436: arg ch for printf verb %s of wrong type: *amqp.Channel
client_test.go:440: arg c for printf verb %s of wrong type: *amqp.Connection
client_test.go:515: arg c for printf verb %s of wrong type: *amqp.Connection
client_test.go:520: arg ch for printf verb %s of wrong type: *amqp.Channel
client_test.go:544: arg c for printf verb %s of wrong type: *amqp.Connection
client_test.go:549: arg ch for printf verb %s of wrong type: *amqp.Channel
connection.go:564: unreachable code
connection.go:699: unreachable code
consumers.go:30: makeConsumers returns Lock by value: amqp.consumers
```

This commit fixes them.