Parameter anon ignored if set to False

[adam-roughton]

The change introduced by #437 has meant that the anon parameter is effectively ignored if set to False: AzureBlobFileSystem(account_name, anon=False)

Should be something like this:

self.anon = anon if anon is not None else os.getenv("AZURE_STORAGE_ANON", "true").lower() not in [
    "false",
    "0",
    "f",
]

or perhaps more readable:

if anon is not None:
   self.anon = anon
else:
   self.anon = os.getenv("AZURE_STORAGE_ANON", "true").lower() not in [
      "false",
      "0",
      "f",
  ]