Re: [url] URL comparison methods (#20)

So, instead of "subsumes", it might be easier to talk about some URL being "within scope" of some other URL. Testing Chrome's service worker implementation,  if URL B's `.pathname` starts with URL's A's `.pathname`, it is "within scope":

So: 
```
//the scope algorithm could be:
URL.inScope = function(a,b){
   if (a === undefined || b === undefined){
      return false; 
   }
   //either A or B can throw. 
   var A = new URL(String(a), window.location); 
   var B = new URL(String(b), window.location);
   return B.pathname.startsWith(A.pathname); 
}

//Given `a` as defining the scope 
a = "/t";

//These are all in scope of `a`
["/test-test/t", 
"/test",
"/test-test",
"/test-test/test"].every(i => URL.inScope(a, i)); //true

//These are all out of scope
[ "z",
 undefined,
 "z-test",
 "xyz123"
].every(i => !URL.inScope(a, i)); //true
```






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https://github.com/webspecs/url/issues/20#issuecomment-67912390