Detect double-signing URLs

[TomAugspurger]

If you accidentally sign a URL twice, you'll get a URL that can't be opened by rasterio. I wonder if we can detect this case and either raise / warn / silently not double sign?

>>> import pystac
>>> import rasterio
>>> import planetary_computer as pc

>>> item = pystac.Item.from_file("https://planetarycomputer.microsoft.com/api/stac/v1/collections/sentinel-2-l2a/items/S2A_MSIL2A_20210803T062221_R048_T40GCP_20210803T154836")
>>> signed_item = pc.sign(item)

>>> rasterio.open(pc.sign(signed_item).assets["B02"].href)

RasterioIOError: HTTP response code: 403