Fix Proc with arity one in param values

[timothysu]

Recently ran into an issue where having custom validations in values would cause documentation generation to fail. This simply mitigates an error from occurring.

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From https://github.com/ruby-grape/grape/blob/master/README.md :

Alternatively, a Proc with arity one (i.e. taking one argument) can be used to explicitly validate
each parameter value. In that case, the Proc is expected to return a truthy value if the parameter
value is valid.

params do
  requires :number, type: Integer, values: ->(v) { v.even? && v < 25 }
end

[coveralls]

Coverage remained the same at 97.642% when pulling eabe736 on salsify:bugfix/values-proc-with-arg into 91e8210 on ruby-grape:master.

[LeFnord]

thanks @timothysu and sorry for the late mörge