Since 2.0.4 calc.solve(a: "a") returns {:a=>"a"} instead of {:a=>:undefined}

[vlazar]

In Dentaku 2.0.3 I see this result, which makes sense:

Dentaku::Calculator.new.solve(a: "a") # => {:a=>:undefined}

Since Dentaku 2.0.4 however it is now

Dentaku::Calculator.new.solve(a: "a") # => {:a=>"a"}

Which seems incorrect behavior to me.

I've tracked it down to this commit 63d2d19

Reverting this particular change gives me an old behavior 63d2d19#diff-fd36f8c62ecf1332daaf88166462fbaa808ab862a15c682bae5b6e873910b5e2L42-R43

[vlazar]

Fixed in #228