ValueMappingEstimator KeyType metadata maps to Key data and not to the Value data

[singlis]

Issue

Discovered while fixing #2083.

When creating a ValueMappingEstimator with KeyTypes as the values, the generated KeyType does not covert back to the original value. This is demonstrated when appending a KeyToValueTransform to retrieve the original values using the following code:

            // Creating a list of keys that are based on the Education values
            // These lists are created by hand for the demonstration, but the ValueMappingEstimator can take in any IEnumerables.
            var educationKeys = new List<string>()
            {
                "0-5yrs",
                "6-11yrs",
                "12+yrs"
            };

            var educationValues = new List<string>()
            {
                "Cat1",
                "Cat2", 
                "Cat3"
            };

            // Generate the estimator with the key type set as true. Even though are values are strings, this will
            // create a key for each value. For this example, a KeyToValue Estimator is added to the ValueMapping Estimator to demonstrate the
            // reverse lookup of the KeyType
            var pipeline = new ValueMappingEstimator<string, string>(ml, educationKeys, educationValues, true, ("Education", "EducationKeyType"))
                              .Append(new KeyToValueMappingEstimator(ml, ("EducationKeyType", "EducationCategory")));

The expected results should be:

Age   Education   EducationCategory
26      0-5yrs          Cat1
42      0-5yrs          Cat1
39      12+yrs          Cat3
34      0-5yrs          Cat1
35      6-11yrs         Cat2

The actual results are:

Age     Education    EducationCategory
26      0-5yrs          0-5yrs
42      0-5yrs          0-5yrs
39      12+yrs          12+yrs
34      0-5yrs          0-5yrs
35      6-11yrs         6-11yrs