Fix use of floatmax

[devmotion]

Fixes #475.

The docstring of floatmax only mentions and shows its application to types, not to values. Hence IMO as in other places in SpecialFunctions we should apply floatmax to types, not to values.

I didn't want to add a test with ForwardDiff since this would create a dependency cycle - ForwardDiff depends on SpecialFunctions, so e.g. breaking releases of SpecialFunctions could not be tested before a new version of ForwardDiff is released. Maybe a test should be added in ForwardDiff once the issue is fixed?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.11%. Comparing base (f8fd782) to head (7a1d2d6).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #494   +/-   ##
=======================================
  Coverage   94.11%   94.11%           
=======================================
  Files          14       14           
  Lines        2905     2905           
=======================================
  Hits         2734     2734           
  Misses        171      171           

Flag	Coverage Δ
unittests	94.11% <100.00%> (ø)

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[mancellin]

Thanks @devmotion for taking care of it