Fix link + docs (#2476)

* Fix a copy/paste fail

Also, fix some incorrect tests.

* Clarify how routes are diffed, improve key + route resolver docs

- Add some missing links to route resolvers and single-child keyed
  fragments, clarify usage around them.
- Drive-by: remove a redundant sentence that itself was missing a
  period.

* Actually test for propagation and preventDefault

Previously, the mocks were both junk and inaccurate. No wonder my tests
were silently failing - they were wrong and not obviously wrong.

api/router.js

@@ -237,17 +237,18 @@ module.exports = function($window, mountRedraw) {
 					// link target, etc. Nope, this isn't just for blind people.
 					if (
 						// Skip if `onclick` prevented default
-						result === false || !e.defaultPrevented &&
+						result !== false && !e.defaultPrevented &&
 						// Ignore everything but left clicks
 						(e.button === 0 || e.which === 0 || e.which === 1) &&
 						// Let the browser handle `target=_blank`, etc.
 						(!e.currentTarget.target || e.currentTarget.target === "_self") &&
 						// No modifier keys
 						!e.ctrlKey && !e.metaKey && !e.shiftKey && !e.altKey
-					) return
-					e.preventDefault()
-					e.redraw = false
-					route.set(href, null, options)
+					) {
+						e.preventDefault()
+						e.redraw = false
+						route.set(href, null, options)
+					}
 				}
 			}
 			return child