context builder should not use create a timeout 0 context when parent context is provided

[AllenLuUber]

In the current ContextBuilder implementation, providing a Parent context but not Timeout options will result in a 0 default value in result Context.

ctx := context.WithTimeout(context.Background(), time.Second)
ctxBuilder := &tchannel.ContextBuilder{
    ParentContext: ctx,
}
// ctxBuilder.Build will return a context with timeout 0