Fix set_description with tqdm.notebook(disable=True)

[mihaild]

As self.sp() is called in other places as well (for example if iterable raises exception), I think it's safer to allow calling it even for disabled instances.

[codecov-io]

Codecov Report

Merging #669 into master will increase coverage by 13.62%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master     #669       +/-   ##
===========================================
+ Coverage   85.56%   99.19%   +13.62%     
===========================================
  Files          15       10        -5     
  Lines         977      748      -229     
  Branches      169      132       -37     
===========================================
- Hits          836      742       -94     
+ Misses        136        2      -134     
+ Partials        5        4        -1

[codecov-io]

Codecov Report

Merging #669 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #669      +/-   ##
==========================================
+ Coverage   99.19%   99.19%   +<.01%     
==========================================
  Files          10       10              
  Lines         747      748       +1     
  Branches      132      132              
==========================================
+ Hits          741      742       +1     
  Misses          2        2              
  Partials        4        4

[casperdcl]

• related: exception and thread safety #548
• fixes 'tqdm_notebook.set_description()' fails with disable=True #650