[PATCH] Array.join making safe strings unsafe

When calling join on an Array comprised entirely of html_safe objects, and using an html_safe separator, the string returned is unsafe. This seems to be a bug, and has broken many of our own custom helpers without providing any additional safety.

I overrode Array.join to return safe strings (SafeBuffers) when all the elements of the array are safe, and the separator is either not specified or html_safe itself. This is consistent with Ruby's behavior when joining arrays of untainted objects with an untainted separator.

Additionally, I overrode html_safe? on Array to return true if every element is safe. This will allow Array.join to operate properly in the recursive case (joining an Array of arrays).