Prohibit writing invalid short_values to GTFS db with JdbcTableWriter

[landonreed]

Some recent test code written by @binh-dam-ibigroup in #312 revealed that any value can be written to a GTFS record using JdbcTableWriter: https://github.com/conveyal/gtfs-lib/blob/a45e6f93239fc66548903fa39abb26ffe35a8915/src/test/java/com/conveyal/gtfs/loader/JDBCTableWriterTest.java#L495-L499

This should be prohibited and the tests updated.