Clarify pathname docs, follow spec with fragments (#2448)

* Clarify pathname docs, follow spec with fragments

- Valid URLs must not contain a `#` within its fragment.
  https://github.com/MithrilJS/mithril.js/issues/2445
- Our docs were a little confusing and misleading - `m.pathname` isn't
  aware of URLs, just path names.
- Removed the relevant extension to `m.parseQueryString` required to
  support the hash parsing extension. Now we just shave it off and
  ignore it.
- Fix support for arbitrary prefixes, so prefixes like `?#` are
  handled correctly.
- Add a bunch of tests to cover various areas of confusion and unusual
  edge cases.

* Update with PR [skip ci]

pathname/parse.js

@@ -9,16 +9,16 @@ module.exports = function(url) {
 	var queryEnd = hashIndex < 0 ? url.length : hashIndex
 	var pathEnd = queryIndex < 0 ? queryEnd : queryIndex
 	var path = url.slice(0, pathEnd).replace(/\/{2,}/g, "/")
-	var params = {}
 
 	if (!path) path = "/"
 	else {
 		if (path[0] !== "/") path = "/" + path
 		if (path.length > 1 && path[path.length - 1] === "/") path = path.slice(0, -1)
 	}
-	// Note: these are reversed because `parseQueryString` appends parameters
-	// only if they don't exist. Please don't flip them.
-	if (queryIndex >= 0) parseQueryString(url.slice(queryIndex + 1, queryEnd), params)
-	if (hashIndex >= 0) parseQueryString(url.slice(hashIndex + 1), params)
-	return {path: path, params: params}
+	return {
+		path: path,
+		params: queryIndex < 0
+			? {}
+			: parseQueryString(url.slice(queryIndex + 1, queryEnd)),
+	}
 }