allow access to spheroid parameter

Hi Minglei,

I have another change. I wanted to access the spheroid parameter to return the distance in meters between two SRID 4326 coordinates.

I've implemented in such a way that if you don't provide the spheroid parameter, it will not provide any value to postgres, thus not interfering with Postgres's default behavior of auto-enabling spheroid if a geography type is provided.

The tests do not conform so closely with your pattern, but I tried to make it match as more than not.

Thank you!