net/mail: fix build.

Change-Id: I8f5c72c6c0db015c06d564523bab35d97d934578
Reviewed-on: https://go-review.googlesource.com/13510Reviewed-by: Michael McGreevy <mcgreevy@golang.org>
Reviewed-by: David Symonds <dsymonds@golang.org>

src/net/mail/message.go

@@ -177,7 +177,7 @@ func (a *Address) String() string {
 		// treat the whole address as local-part.
 		local = a.Address
 	} else {
-		local, domain := a.Address[:at], a.Address[at+1:]
+		local, domain = a.Address[:at], a.Address[at+1:]
 	}
 
 	// Add quotes if needed

src/net/mail/message_test.go

@@ -485,7 +485,7 @@ func TestAddressFormatting(t *testing.T) {
 		},
 		{ // https://golang.org/issue/12098
 			&Address{Name: "Rob", Address: ""},
-			`"Rob" <>`,
+			`"Rob" <@>`,
 		},
 		{ // https://golang.org/issue/12098
 			&Address{Name: "Rob", Address: "@"},