* core: fix crash with no plugins

A Corefile that defines a zone without plugins crashes coredns with the
stack trace below. Change this to return a refused.

~~~ corefile
 example.org {
    whoami
    log
    cache
    debug
}

example.net {
}
~~~

Asking for anyhing in example.net does this. Add test that tests this.

~~~
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0xa5e6a4]

goroutine 55 [running]:
github.com/coredns/coredns/core/dnsserver.(*Server).ServeDNS(0xc000438f60, 0x2059420, 0xc0005a4030, 0x206c0e0, 0xc000522140, 0xc0005ae000)
	/home/miek/src/github.com/coredns/coredns/core/dnsserver/server.go:247 +0x884
github.com/coredns/coredns/core/dnsserver.(*Server).ServePacket.func1(0x206dd00, 0xc00009e180, 0xc0005ae000)
	/home/miek/src/github.com/coredns/coredns/core/dnsserver/server.go:126 +0xaf
github.com/miekg/dns.HandlerFunc.ServeDNS(0xc000529270, 0x206dd00, 0xc00009e180, 0xc0005ae000)
	/home/miek/go/pkg/mod/github.com/miekg/dns@v1.1.31/server.go:37 +0x44
github.com/miekg/dns.(*Server).serveDNS(0xc000286c60, 0xc000282400, 0x34, 0x200, 0xc00009e180)
	/home/miek/go/pkg/mod/github.com/miekg/dns@v1.1.31/server.go:609 +0x2f7
github.com/miekg/dns.(*Server).serveUDPPacket(0xc000286c60, 0xc0003b03b4, 0xc000282400, 0x34, 0x200, 0xc00000e320, 0xc000522080)
	/home/miek/go/pkg/mod/github.com/miekg/dns@v1.1.31/server.go:549 +0xb2
created by github.com/miekg/dns.(*Server).serveUDP
	/home/miek/go/pkg/mod/github.com/miekg/dns@v1.1.31/server.go:479 +0x292
~~~

Also fix single typo in chaos_test.go
Signed-off-by: Miek Gieben <miek@miek.nl>

* Fix naming
Signed-off-by: Miek Gieben <miek@miek.nl>