Add possibility of passing arguments to XMLHttpRequest constructor

[Veneti]

Making AJAX requests on Firefox OS (operating system, not regular browser) does not work unless XMLHttpRequest is created in the following way:

var xhr = new window.XMLHttpRequest({mozSystem:true});

What makes difference is 'mozSystem'. In another ticket #13394 someone proposed setting it via xhrFields however mozSystem is read-only and can not be set once XMLHttpRequest object is created. It can be set only on creation:

​https://developer.mozilla.org/en-US/docs/DOM/XMLHttpRequest

[jaubourg]

You can already handle the problem 3 different ways:

1. global override, no condition:

   $.ajaxSetup( {
     xhr: function() {
       return new window.XMLHttpRequest( {
         mozSystem: true
       } );
     }
   } );
   

2. local override, simple condition:

   $.ajaxPrefilter( function( options ) {
     if ( options.firefoxOS ) {
       options.xhr = function() {
         return new window.XMLHttpRequest( {
           mozSystem: true
         } );
       }
     }
   } );
   
   // Then later on
   
   $.ajaxSetup( {
     firefoxOS: true
   } );
   

3. Generic constructor parameter

   $.ajaxPrefilter( function( options ) {
     if ( options.xhrConstructParam ) {
       options.xhr = function() {
         return new window.XMLHttpRequest( options.xhrConstructParam );
       }
     }
   } );
   
   // Then later on
   
   $.ajaxSetup( {
     xhrConstructParam: {
       mozSystem: true
     }
   } );
   

1. is simpler but cannot be used generically in a project that may run outside of FirefoxOS,
2. is better at isolating FirefoxOS specific code,
3. is to be used for a more generic approach.

Anyway, we should probably consider adding a standard option (as in 3) seeing as the options object param is part of the standard now: ​http://www.w3.org/TR/XMLHttpRequest/#dom-xmlhttprequest

[dmethvin]

Method 1 seems good enough here? If we're going to add more code we need docs before landing it, and I shudder to think of more docs for ajax.

[dmethvin]

Several good solutions were given above so it seems like we can avoid another mechanism and the docs it would require.