Fixed error messages in TestDefault

[ablankz]

A partial code of cors_test.go is shown below.

if s.allowedHeaders == nil {
    t.Error("c.allowedHeaders should be nil when Default")
}
if s.allowedMethods == nil {
    t.Error("c.allowedMethods should be nil when Default")
}

We believe the following message is appropriate for this.

if s.allowedHeaders == nil {
    t.Error("c.allowedHeaders must not be nil when Default")
}
if s.allowedMethods == nil {
    t.Error("c.allowedMethods must not nil when Default")
}