Make ST_Transform return a MULTILINESTRING for MULTILINESTRING input

This is about #307. I force `ST_Transform` of a `MULTILINESTRING` to be a `MULTILINESTRING`, even
though JTS doesn't do this. This is particularly useful for exporting to SHP.

I also clean up `ST_Transform`'s unit tests:

* make them more readable
* fix typos
* always check output SRID
* add 2 unit tests