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:

gtfs-lib/src/test/java/com/conveyal/gtfs/loader/JDBCTableWriterTest.java

Lines 495 to 499 in a45e6f9

	pattern_stop.timepoint = 2;
	pattern_stop.drop_off_type = 3;
	pattern_stop.pickup_type = 4;
	pattern_stop.continuous_pickup = 5;
	pattern_stop.continuous_drop_off = 6;

This should be prohibited and the tests updated.