patch to not re-code geo field unless the source has changed

Saved an edited node causes the geocode widget to recode its field, even if the source field hasn't been modified. Since this geocoding can sometimes take a long time, I've added a one-line check to see if the source field hasn't been modified (checks the file ID), in which case the geo re-coding is skipped. It works with single or multiple instances of the source field.

Hoping to get it committed.

Thanks, and peace...

Shiraz