Fixed error in extending MutableBitmap

[jorgecarleitao]

This error was again derived from assuming that the bits were unset, which is no longer true with extend_from_slice.

[codecov]

Codecov Report

Merging #393 (f266051) into main (a238efe) will increase coverage by 0.00%.
The diff coverage is 90.90%.

@@           Coverage Diff           @@
##             main     #393   +/-   ##
=======================================
  Coverage   81.13%   81.13%           
=======================================
  Files         347      347           
  Lines       21993    21991    -2     
=======================================
- Hits        17843    17842    -1     
+ Misses       4150     4149    -1     

Impacted Files	Coverage Δ
src/bitmap/mutable.rs	89.27% <83.33%> (+0.10%)	⬆️
tests/it/bitmap/mutable.rs	100.00% <100.00%> (ø)

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