strings: disallow ':' in nicks

This matches the behavior of inspircd at the very least.

Previously, the comment above that section claimed ':' should be
disallowed, but the code didn't do so.

I also simplified the code a little bit and added tests.

irc/strings_test.go

@@ -0,0 +1,105 @@
+// Copyright (c) 2017 Euan Kemp
+// released under the MIT license
+
+package irc
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestCasefoldChannel(t *testing.T) {
+	type channelTest struct {
+		channel string
+		folded  string
+		err     bool
+	}
+	testCases := []channelTest{
+		{
+			channel: "#foo",
+			folded:  "#foo",
+		},
+		{
+			channel: "#rfc1459[noncompliant]",
+			folded:  "#rfc1459[noncompliant]",
+		},
+		{
+			channel: "#{[]}",
+			folded:  "#{[]}",
+		},
+		{
+			channel: "#FOO",
+			folded:  "#foo",
+		},
+		{
+			channel: "#bang!",
+			folded:  "#bang!",
+		},
+		{
+			channel: "#",
+			folded:  "#",
+		},
+	}
+
+	for _, errCase := range []string{
+		"", "#*starpower", "# NASA", "#interro?", "OOF#", "foo",
+	} {
+		testCases = append(testCases, channelTest{channel: errCase, err: true})
+	}
+
+	for i, tt := range testCases {
+		t.Run(fmt.Sprintf("case %d: %s", i, tt.channel), func(t *testing.T) {
+			res, err := CasefoldChannel(tt.channel)
+			if tt.err {
+				if err == nil {
+					t.Errorf("expected error")
+				}
+				return
+			}
+			if tt.folded != res {
+				t.Errorf("expected %v to be %v", tt.folded, res)
+			}
+		})
+	}
+}
+
+func TestCasefoldName(t *testing.T) {
+	type nameTest struct {
+		name   string
+		folded string
+		err    bool
+	}
+	testCases := []nameTest{
+		{
+			name:   "foo",
+			folded: "foo",
+		},
+		{
+			name:   "FOO",
+			folded: "foo",
+		},
+	}
+
+	for _, errCase := range []string{
+		"", "#", "foo,bar", "star*man*junior", "lo7t?",
+		"f.l", "excited!nick", "foo@bar", ":trail",
+		"~o", "&o", "@o", "%h", "+v", "-m",
+	} {
+		testCases = append(testCases, nameTest{name: errCase, err: true})
+	}
+
+	for i, tt := range testCases {
+		t.Run(fmt.Sprintf("case %d: %s", i, tt.name), func(t *testing.T) {
+			res, err := CasefoldName(tt.name)
+			if tt.err {
+				if err == nil {
+					t.Errorf("expected error")
+				}
+				return
+			}
+			if tt.folded != res {
+				t.Errorf("expected %v to be %v", tt.folded, res)
+			}
+		})
+	}
+}