Storage SAS Token from API does not match Azure Portal

### Which service(blob, file, queue) does this issue concern?
Requirement is for `blob` only

### Which version of the SDK was used? Please provide the output of `pip freeze`.
`azure-storage-blob==1.4.0`

### What problem was encountered?
When comparing the output of `azure.storage.blob.BlockBlobService.generate_account_shared_access_signature()` with the same parameters, the SAS token generated does not match what the Azure Portal generates.

`bbs_acct_token = bbs.generate_account_shared_access_signature(
ResourceTypes.CONTAINER + ResourceTypes.OBJECT,
AccountPermissions.READ + AccountPermissions.WRITE + AccountPermissions.LIST \
# + AccountPermissions.CREATE,
,datetime.utcnow() + timedelta(hours=1))`

Generates: `se=2019-07-28T16%3A07%3A27Z&sp=&sv=2017-04-17&ss=b&srt=co&sig=%2BTO1J2/zNIYlvhEonRkFRTZPkNvm...`

*There seems to be an issue with the `sp=` value - am expecting `rwl` but there is nothing.

From the Azure Portal (using the same parameters) the SAS Token looks like:

`?sv=2018-03-28&ss=b&srt=co&sp=rwdlac&se=2019-07-28T22:45:47Z&st=2019-07-28T14:45:47Z&spr=https&sig=PMu3KnDRV7VzOig1ewVP2T4TU0qowmEn...`

### Have you found a mitigation/solution?
No - the requirement is to generate the token thru the storage API