Correctly group R53_ALIAS records during IncrementalDiff. (#399)

Previously, unnecessary corrections were possible if both an R53_ALIAS
pointing to an A record and to an AAAA record existed for the same label,
and map iteration over existing and desired found them in different orders.
(This is a common configuration for IPv6-enabled records.)

This commit:
 * mirrors key logic in the R53 provider
 * centralizes logic around keys in the models package
 * adds tests

models/record_test.go

@@ -0,0 +1,33 @@
+package models
+
+import "testing"
+
+func TestKey(t *testing.T) {
+	var tests = []struct {
+		rc       RecordConfig
+		expected RecordKey
+	}{
+		{
+			RecordConfig{Type: "A", Name: "@"},
+			RecordKey{Type: "A", Name: "@"},
+		},
+		{
+			RecordConfig{Type: "R53_ALIAS", Name: "@"},
+			RecordKey{Type: "R53_ALIAS", Name: "@"},
+		},
+		{
+			RecordConfig{Type: "R53_ALIAS", Name: "@", R53Alias: map[string]string{"foo": "bar"}},
+			RecordKey{Type: "R53_ALIAS", Name: "@"},
+		},
+		{
+			RecordConfig{Type: "R53_ALIAS", Name: "@", R53Alias: map[string]string{"type": "AAAA"}},
+			RecordKey{Type: "R53_ALIAS_AAAA", Name: "@"},
+		},
+	}
+	for i, test := range tests {
+		actual := test.rc.Key()
+		if test.expected != actual {
+			t.Errorf("%d: Expected %s, got %s", i, test.expected, actual)
+		}
+	}
+}