Fixes #67

This PR fixes #67 where calling load() on a loading=lazy element causes the contents not to be replaced when the element becomes visible.

Context

There are two bugs at play here:

1. We were calling observer.unobserve when load() was called. This is incorrect as it should only be called once content replacement has happened.
2. load() doesn't actually properly preload the request, since nothing is done with the response - it should be saved into the weakmap but that is only done by getData() not load().

Fix

The fix here is twofold:

1. Remove the observer.unobserve call from the load() code.
2. Move load() logic to a private function and instead call getData from the element.load() function, getData correctly caches the response or immediately returns a cached response, so it is the correct code to call to preload the response#66

Implications

Technically this change does modify the public api of include-fragment - if someone was subclassing the element and re-implementing load() to modify say how events are fired, this will no longer work for them. I cannot find any examples of us doing this within github (we do reimplement fetch but not load.