Heatmap doesn't work when dates are used in columns

## Bug Report

### Steps to Reproduce:
 1.  run `remote_bug4.py` from the attached archive
[date_column_bug.zip](https://github.com/finos/perspective/files/4938871/date_column_bug.zip)

 2. open `http://localhost:8888` in the browser, change plugin to 'Heatmap and observe the following
<img width="1858" alt="image" src="https://user-images.githubusercontent.com/4868370/87808358-3d5bd480-c862-11ea-86fb-783ea9e550be.png">

 3. Press on

<img width="25" alt="image" src="https://user-images.githubusercontent.com/4868370/87808381-477dd300-c862-11ea-9957-7b280c49fb6d.png">

and observe the following

<img width="1864" alt="image" src="https://user-images.githubusercontent.com/4868370/87808402-52386800-c862-11ea-9b66-a23c52f169b7.png">

### Environment:
Perspective 0.5.1



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Heatmap doesn't work when dates are used in columns #1132

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