fixed #2082 modify codes of cc.setBlending to fit WebGL's bug
Merge branch 'master' of https://github.com/cocos2d/cocos2d-html5 into Iss2082_FixSomeBugs
Merge pull request #906 from dingpinglv/Iss2082_FixSomeBugs

fixed #2082 modify codes of cc.setBlending to fit WebGL's bug