Accept MethodType as panel.serve parameter

[nritsche]

Closes #1449

[nritsche]

I didn't find an easy way to add a test for this, but it makes my panel application work.

[codecov]

Codecov Report

Merging #1450 into master will decrease coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1450      +/-   ##
==========================================
- Coverage   87.89%   87.78%   -0.12%     
==========================================
  Files         135      135              
  Lines       14881    14930      +49     
==========================================
+ Hits        13080    13106      +26     
- Misses       1801     1824      +23     

Impacted Files	Coverage Δ
panel/io/server.py	62.63% <100.00%> (ø)
panel/pane/plot.py	63.77% <0.00%> (-11.49%)	⬇️
panel/tests/test_param.py	99.86% <0.00%> (+<0.01%)	⬆️
panel/param.py	91.59% <0.00%> (+0.03%)	⬆️

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[philippjfr]

Thanks! Don't worry about the test for now, I'll also have to set a higher threshold on CodeCov.