Nucleotide-count: DNA has no uracil

[soniakeys]

It's surprising to me that count('U') should return 0 rather than throw an error. The README doesn't mention U specifically and doesn't suggest this special case to me.

[aarti]

@soniakeys could you please specify the exercise name.

[soniakeys]

Oh sorry, this is in nucleotide-count.

[kytrinyx]

The readme used to mention uracil, but it's a pretty bad idea to mix uracil into everything. I meant to update all the exercises this weekend, but got distracted.

[kytrinyx]

I updated the ruby problem, and I'm adding issues for it in the other languages.

I also changed the API of the problem in Ruby so that there is no error checking in the object itself, only in the factory that creates it: https://github.com/exercism/xruby/blob/master/nucleotide-count/example.rb#L2-L8

I don't know if that's relevant or idiomatic in Lua.