Fix Location sync serialization

[philippjfr]

Fixes #1784

Cc: @hoxbro Sorry for closing your PR but using param serialization correctly is non-obvious so this was going to result in a quicker turnaround.

[hoxbro]

No problem. I was suspecting that there were a better way.

I also hope it is okay that I was "half-fixing" a problem like this, to be better to understand the underlying framework.

[codecov]

Codecov Report

Merging #1792 (0d6e1a7) into master (f22e267) will increase coverage by 0.00%.
The diff coverage is 83.33%.

@@           Coverage Diff           @@
##           master    #1792   +/-   ##
=======================================
  Coverage   85.13%   85.13%           
=======================================
  Files         151      151           
  Lines       17923    17936   +13     
=======================================
+ Hits        15258    15270   +12     
- Misses       2665     2666    +1     

Impacted Files	Coverage Δ
panel/io/location.py	85.24% <83.33%> (+0.84%)	⬆️

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[philippjfr]

Note this actually causes another regression because param serialization of tuples was broken, see holoviz/param#446