Error when metadata to `get_numerical_dates()` is not a pandas DataFrame

[victorlin]

Description of proposed changes

See commit messages and #1025.

Related issue(s)

• Fixes #1025

Testing

• [x] Added 84a4b0397e90c80947a48ab9f1e811382933f323 which fails, then fixed it with the following commit.

Checklist

• [x] Add a message in CHANGES.md summarizing the changes in this PR. Keep headers and formatting consistent with the rest of the file.

[codecov[bot]]

Codecov Report

Base: 61.64% // Head: 61.68% // Increases project coverage by +0.03% :tada:

Coverage data is based on head (918fe65) compared to base (f97b1cb). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1026      +/-   ##
==========================================
+ Coverage   61.64%   61.68%   +0.03%     
==========================================
  Files          52       52              
  Lines        6286     6287       +1     
  Branches     1583     1583              
==========================================
+ Hits         3875     3878       +3     
+ Misses       2139     2138       -1     
+ Partials      272      271       -1     

Impacted Files	Coverage Δ
augur/dates.py	92.85% <100.00%> (+3.00%)	:arrow_up:

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