contract.call() block_identifier default change

[newtectonics]

Changed default block_identifier in contract.call() to None.
Changed parse_block_identifier to use web3.eth.defaultBlock if None is passed in

What was wrong?

#2334

How was it fixed?

changed block_identifier default to None in contract.call
changed parse_block_identifier to parse None to web3.eth.default_block

Todo:

• Add entry to the release notes

Cute Animal Picture

[kclowes]

Thanks for the PR! This looks like a good change to me, but we'll need to wait until v6 to pull it in since it's breaking like you mentioned in the issue. We're planning on cutting our v6 branch soon, so will add this to be pulled in then. It will need a test before merging. If you have time to add one that would be great, otherwise I should be able to get to it soon:tm:.

[kclowes]

Sorry @newtectonics that I wasn't able to keep this PR. I opened one at #2777 that has your initial commit, and I added some tests and things after that. You should still get credit on that initial commit.

Closing in favor of #2777.