Fix return value for Boolean::process().

[ADmad]

Closes #308

[codecov]

Codecov Report

Merging #309 (f128a1b) into master (2523851) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #309      +/-   ##
============================================
+ Coverage     93.36%   93.37%   +0.01%     
  Complexity      233      233              
============================================
  Files            19       19              
  Lines           603      604       +1     
============================================
+ Hits            563      564       +1     
  Misses           40       40              

Impacted Files	Coverage Δ	Complexity Δ
src/Model/Filter/Boolean.php	100.00% <100.00%> (ø)	9.00 <0.00> (ø)

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