feat(blob.py): auto-populate standard headers for non-chunked downloads

[william-silversmith]

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Fixes #24 🦕

This PR autopopulates the following fields for non-chunked downloads based on the server header response:

blob.content_encoding
blob.content_type
blob.cache_control
blob.storage_class
blob.content_language
blob.md5_hash
blob.crc32c

[tseaver]

@william-silversmith Thanks very much for the patch! In addition to tweaking the merge conflict, we need to add unit tests and / or assertions to show that the various properties are set / cleared based on the response payload.

[william-silversmith]

@tseaver would these unit tests be sufficient? I can get more involved if necessary. I had trouble invoking _do_download, it was interacting strangely with "MagicMock" and dying on closing a file stream.

[tseaver]

@william-silversmith Thanks, that new test was sufficient.

[william-silversmith]

I'm a little confused because 806-808 have to be evaluated for the crc32c and md5_hash tests to pass.

[google-cla]

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[tseaver]

@googlebot I consent

[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[william-silversmith]

@googlebot I fixed it.

[google-cla]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[william-silversmith]

I could try force pushing a double quote update that removes you as an author if that would be okay with you?

[tseaver]

@william-silversmith Sure, WFM.

[tseaver]

W00t! Thanks for persevering, @william-silversmith .

[william-silversmith]

Excited thank you!