Continuing to map request headers using their name in uppercase for the index,...

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.js

@@ -416,7 +416,7 @@ jQuery.extend({
 				// Caches the header
 				setRequestHeader: function( name, value ) {
 					if ( !state ) {
-						requestHeaders[ name.toLowerCase().replace( rucHeaders, rucHeadersFunc ) ] = value;
+						requestHeaders[ name.toUpperCase().replace( rucHeaders, rucHeadersFunc ) ] = { name: name, value: value};
 					}
 					return this;
 				},
@@ -664,24 +664,27 @@ jQuery.extend({
 
 		// Set the correct header, if data is being sent
 		if ( s.data && s.hasContent && s.contentType !== false || options.contentType ) {
-			requestHeaders[ "Content-Type" ] = s.contentType;
+			jqXHR.setRequestHeader( "Content-Type", s.contentType );
 		}
 
 		// Set the If-Modified-Since and/or If-None-Match header, if in ifModified mode.
 		if ( s.ifModified ) {
 			ifModifiedKey = ifModifiedKey || s.url;
 			if ( jQuery.lastModified[ ifModifiedKey ] ) {
-				requestHeaders[ "If-Modified-Since" ] = jQuery.lastModified[ ifModifiedKey ];
+				jqXHR.setRequestHeader( "If-Modified-Since", jQuery.lastModified[ ifModifiedKey ] );
 			}
 			if ( jQuery.etag[ ifModifiedKey ] ) {
-				requestHeaders[ "If-None-Match" ] = jQuery.etag[ ifModifiedKey ];
+				jqXHR.setRequestHeader( "If-None-Match", jQuery.etag[ ifModifiedKey ] );
 			}
 		}
 
 		// Set the Accepts header for the server, depending on the dataType
-		requestHeaders.Accept = s.dataTypes[ 0 ] && s.accepts[ s.dataTypes[0] ] ?
-			s.accepts[ s.dataTypes[0] ] + ( s.dataTypes[ 0 ] !== "*" ? ", */*; q=0.01" : "" ) :
-			s.accepts[ "*" ];
+                jqXHR.setRequestHeader(
+					"Accept",
+					s.dataTypes[ 0 ] && s.accepts[ s.dataTypes[0] ] ?
+						s.accepts[ s.dataTypes[0] ] + ( s.dataTypes[ 0 ] !== "*" ? ", */*; q=0.01" : "" ) :
+						s.accepts[ "*" ]
+					);
 
 		// Check for headers option
 		for ( i in s.headers ) {

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( _ ) {}