Standardized the type checks across core. isFunction and isArray now use Object.proto...

Standardized the type checks across core. isFunction and isArray now use Object.prototype.toString to verify the type, .constructor use was removed in favor of typeof, typeof checks now use ===, undefined checks use === undefined. All of this is outlined in the new style guidelines: docs.jquery.com/JQuery_Core_Style_Guidelines#Type_Checks. Fixes bug #3618.