CSS: Correct misrepresentation of "auto" horizontal margins as 0

Fixes gh-2237
Closes gh-2276

(cherry picked from commit 214e1634ab)

Conflicts:
	src/css.js
	src/css/support.js
	test/unit/support.js

src/css.js

@@ -350,6 +350,19 @@ jQuery.each( [ "height", "width" ], function( i, name ) {
 	};
 } );
 
+jQuery.cssHooks.marginLeft = addGetHookIf( support.reliableMarginLeft,
+	function( elem, computed ) {
+		if ( computed ) {
+			return ( parseFloat( curCSS( elem, "marginLeft" ) ) ||
+				elem.getBoundingClientRect().left -
+					swap( elem, { marginLeft: 0 }, function() {
+						return elem.getBoundingClientRect().left;
+					} )
+				) + "px";
+		}
+	}
+);
+
 // These hooks are used by animate to expand properties
 jQuery.each( {
 	margin: "",