Continuing to map request headers using their name in uppercase for the index, but we now map to an object with an unmodified name and value: { name: "", value: "" }. This prevents overwriting of the user's supplied headers, which many applications parse with case sensitivity, because we no longer use the modified, uppercase index when building the request.

src/ajax/xhr.js

@@ -104,7 +104,7 @@ if ( jQuery.support.ajax ) {
 					// Need an extra try/catch for cross domain requests in Firefox 3
 					try {
 						for ( i in headers ) {
-							xhr.setRequestHeader( i, headers[ i ] );
+							xhr.setRequestHeader( headers[ i ].name, headers[ i ].value );
 						}
 					} catch( _ ) {}