missing value on insert for STI entities

[webleedev]

Issue type:

[ ] question
[x] bug report
[ ] feature request
[ ] documentation issue

Database system/driver:

[ ] cordova
[ ] mongodb
[ ] mssql
[x] mysql / mariadb
[ ] oracle
[ ] postgres
[ ] sqlite
[ ] sqljs
[ ] react-native
[ ] expo

TypeORM version:

[ ] latest
[x] @next
[ ] 0.x.x (or put your version here)

Steps to reproduce or a small repository showing the problem:

1. define entity with child inheritance
2. define shared column for each child entity

notice some entities may not be inserted correctly if all entities used in save operation

[webleedev]

It appears defining various properties on a related seet ofChildEntity with the same column name can cause conflicts

[webleedev]

issue was seen with incorrectly defined column.relationMetadata when column.getEntityValue is called (seen here)