[w3c/manifest] Fix references to lower case in spec (#602)

Opening per [discussion](https://github.com/w3c/manifest/pull/598#issuecomment-321781841) with @kenchris in #598 

The Manifest spec includes three instances of "to lower-case" and two instances of "to ASCII lowercase"

Should all of these be made consistent and updated to reference: https://www.ecma-international.org/ecma-262/8.0/index.html#sec-string.prototype.tolowercase


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