Add base64 padding to networkstatus 'descDigest' field.
Add base64 padding to networkstatus 'descDigest' field.

The trailing '='s in the "descriptor digest" field of an
`@type-bridge-networkstatus` descriptor are removed before placing them
in the descriptor. Therefore, before un-base64-encoding this field, we
must re-add the trailing '='s character padding back in, the same as we
already to for the "identity digest" field on that same descriptor line.

 * FIXES a bug in function `bridgedb.parse.networkstatus.parseRLine()`
   which resulted in the "descriptor digest" field (and all fields after
   it on the 'r'-line) being set to `None`, due to a base64 decoding
   error from missing '=' character padding.

pagea/bridgedb,wfn/bridgedb,pagea/bridgedb,mmaker/bridgedb,wfn/bridgedb,mmaker/bridgedb