fix: respect explicit GPS ref values during write

[lodev09]

Summary

Fixes #12. write() now respects explicit GPSLatitudeRef, GPSLongitudeRef, and GPSAltitudeRef values when provided alongside coordinates, instead of always deriving direction from the sign of the coordinate value.

This fixes the read→write round-trip where GPSLongitudeRef: 'W' with a positive longitude would be incorrectly overwritten to 'E'.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Test Plan

• Long-press library button in example app to run read→write→verify round-trip
• Tested on iOS with a photo containing western longitude — GPSLongitudeRef: 'W' preserved correctly

Checklist

• I tested on iOS
• I tested on Android
• I updated the documentation (if needed)