Updates PigToThrift to support more Pig map (String) key conversions

Very helpful for conversion from pig tuple to thrift struct which includes map with key type `bool`, `byte`, `i16`, `i32`, `i64`, `double`, as well as `string` or `enum`. Type `binary` still not supported due to reliance on thrift < 0.6.0.

This also fixes a ThriftToPig unit test failure I was running into with `mvn test`.