Fix crash when initial selection contains mix of string and bool

[jonmmease]

Fix a crash when a Vega spec contains an initial selection like this:

        [
            {
              "unit": "",
              "fields": [
                {"field": "Major Genre", "channel": "x", "type": "E"},
                {"field": "is_pg", "channel": "color", "type": "E"}
              ],
              "values": ["Adventure", true]
            },
            {
              "unit": "",
              "fields": [
                {"field": "Major Genre", "channel": "x", "type": "E"},
                {"field": "is_pg", "channel": "color", "type": "E"}
              ],
              "values": ["Comedy", false]
            }
        ]

When converting to arrow, the true and false scalars get converted to the string "true" and "false", which resulted in the following error in vlSelectionTest.

'Boolean = Utf8' can't be evaluated because there isn't a common type to coerce the types to

This fixes the crash, by converting the strings "true" and "false" back to booleans when a string is compared to a boolean.