port over fix for testing/replacing a whole document to v4

Handle applying to the whole doc. Fixes evanphx#142

Remove go.mod because I guess I do'nt know how go modules work still

Merge pull request evanphx#141 from kacejot/preserve-nulls

MergeMergePatches preserve nulls in complex objects

Merge pull request evanphx#141 from kacejot/preserve-nulls

MergeMergePatches preserve nulls in complex objects

Fix zero sized document crash

Make it into a real go module and fix CI

Make it into a real go module and fix CI

Merge pull request evanphx#113 from liggitt/4.x-module

Remove 4.x go.mod to make semantic versions addressable by tag

Merge pull request evanphx#108 from liggitt/map-compare

Fix map comparison of nil values and missing keys