Fix lambda error

[newville]

This fixes and adds a test for #148

[codecov]

Codecov Report

❌ Patch coverage is 97.43590% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.4%. Comparing base (abf86ad) to head (7aa10dc).
⚠️ Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
asteval/astutils.py	96.7%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #150   +/-   ##
======================================
  Coverage    95.4%   95.4%           
======================================
  Files           4       4           
  Lines        2182    2233   +51     
======================================
+ Hits         2082    2132   +50     
- Misses        100     101    +1     

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