Add symmetric="true" to \middle results. (mathjax/MathJax#3560)

[dpvc]

This PR makes sure that the character used for \middle is placed symmetrically (to match the open abdominal close delimiters).

This resolves issue mathjax/MathJax#3560

[dpvc]

OOPS, forgot to update the tests. I'll take care of that.

[dpvc]

I've updated the tests, so this can be reviewed now.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.47%. Comparing base (a65456c) to head (f8a3f94).

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #1499     +/-   ##
==========================================
  Coverage    86.47%   86.47%             
==========================================
  Files          340      340             
  Lines        86125    86125             
  Branches      3206     4848   +1642     
==========================================
  Hits         74476    74476             
+ Misses       11649    11628     -21     
- Partials         0       21     +21     

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