Make rspec test pass in Ruby

I have a test for which I need to write the code to make it pass. The test is this:

require 'lib/String.rb'
RSpec.describe String do
  context '.to_h' do
    let(:hash) { { 'hello' => 'tree' } }

    it 'it should return the string parsed as a hash' do
      expect(hash.to_s.gsub('=>', ':').to_h).to eq(hash)
    end

    it 'should raise parse error if there was a parsing error' do
      expect { hash.to_s.to_h }.to raise_error(String::ParseError)
      expect(String::Error).to be < StandardError
      expect(String::ParseError).to be < String::Error
    end
  end
end

The code I wrote up till now is:

class String
    class ParseError < StandardError
        def initialize
            String.const_set("Error", self)
        end
    end

    def to_h
        if self.split(":").count>1 
            eval(self.split(":")[0]+"=>"+self.split(":")[1])
        else
            raise ParseError
        end
    end
end

I must say that I am new to ruby so the code I wrote could be useless.