Support subschema in overwriteByKey
Merge branch 'master' of https://github.com/avian2/jsonmerge

Conflicts:
	jsonmerge/strategies.py
	tests/test_jsonmerge.py
Improve schema generation on arrayMergeById

Conflicts:
	tests/test_jsonmerge.py
Explanation for $ref workaround in arrayMergeById
Add test where get_schema descends twice into $ref
Merge branch 'fix-arrayMergeById-get_schema'