Add Formatting Logic to PublicFolder.Prefix Setter

[scottoffen]

Rather than always pre-pending a '/' to the prefix - which will be problematic if it already starts with a '/' - put logic in the setter to trim white space from the string and remove any leading and trailing slashes before adding a leading slash to the value.

var folder = new PublicFolder();

// Old Behavior
folder.Prefix = "hello"; // Prefix = "hello"

// New Behavior
folder.Prefix = "hello"; // Prefix = "/hello";
folder.Prefix = "/hello"; // Prefix = "/hello";
folder.Prefix = "hello/"; // Prefix = "/hello";
folder.Prefix = "/hello/"; // Prefix = "/hello";
folder.Prefix = "  /hello/  "; // Prefix = "/hello";