Core: Improve isNumeric logic and test coverage

Also add back accidentally deleted comments about the implementation.

Fixes gh-2780
Ref gh-2663
Ref gh-2781
Closes gh-2827

src/core.js

@@ -217,7 +217,11 @@ jQuery.extend( {
 		// that can be coerced to finite numbers (gh-2662)
 		var type = jQuery.type( obj );
 		return ( type === "number" || type === "string" ) &&
-			( obj - parseFloat( obj ) + 1 ) >= 0;
+
+			// parseFloat NaNs numeric-cast false positives ("")
+			// ...but misinterprets leading-number strings, particularly hex literals ("0x...")
+			// subtraction forces infinities to NaN
+			!isNaN( obj - parseFloat( obj ) );
 	},
 
 	isPlainObject: function( obj ) {