Fix incorrect mapping in mixpanel destination

[spjtls9]

A summary of your pull request, including the what change you're making and why.

• width and height were incorrectly mapped to screen field.
• Fix the error: width -> screen.width; height -> screen.height

Testing

Include any additional information about the testing you have completed to
ensure your changes behave as expected. For a speedy review, please check
any of the tasks you completed below during your testing.

• Added unit tests for new functionality
• Tested end-to-end using the local server
• [If destination is already live] Tested for backward compatibility of destination. Note: New required fields are a breaking change.
• [Segmenters] Tested in the staging environment
• [Segmenters] [If applicable for this change] Tested for regression with Hadron.