Fix::On#on

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Add on method to the DSL.

@api public

@example On +2, it must equal 44.
  on(:+, 2) do
    it { MUST equal 44 }
  end

@param method_name [Symbol] The identifier of a method.
@param args        [Array]  A list of arguments.
@param block       [Proc]   A spec to compare against the computed value.

@return [Array] List of results.
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