Readded double escaping into xappSend.

xappSend now escapes the name-value pair to send using encodeURIComponent and then jQuery applies a second
escaping. The server side then has to apply 2 unescapes.

Prior to this, only one escaping was applied - the jQuery escaping. The problem with this is that it
escapes spaces to '+' rather than '%20'. The unescape function in the server side could not handle this.
So, applying an additional escaping before it gets to jQuery (using encodeURIComponent) gets round this problem.