[AMP] Fix upload_date and timestamp extraction

[adrianheine]

Before submitting a pull request make sure you have:

• Searched the bugtracker for similar pull requests
• Read adding new extractor tutorial
• Read youtube-dl coding conventions and adjusted the code to meet them
• Covered the code with tests (note that PRs without tests will be REJECTED)
• Checked the code with flake8

In order to be accepted and merged into youtube-dl each piece of code must be in public domain or released under Unlicense. Check one of the following options:

• I am the original author of this code and I am willing to release it under Unlicense
• I am not the original author of this code but it is in public domain or released under Unlicense (provide reliable evidence)

What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

[remitamine]

BleacherReportCMSIE test is also affected by this change.

[adrianheine]

BleacherReportCMSIE test is also affected by this change.

That video seems to not be available anymore anyway.

[remitamine]

BleacherReportCMSIE test is also affected by this change.

That video seems to not be available anymore anyway.

the extractor does not rely on the webpage.

[adrianheine]

the extractor does not rely on the webpage.

You're right, thanks for your diligence.