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## Environment
**Liquibase Version**:  4.1

**Liquibase Integration & Version**: gradle

**Liquibase Extension(s) & Version**: 

**Database Vendor & Version**: any 

**Operating System Type & Version**: Win10, Java8

## Description
If you add two dbms to a changeset, it seperates them with ',xxx' instead of ','. Therefore the second dbms will not be correctly recognized.

 `<changeSet  dbms="oracle,xxxmssql" ... >`

## Steps To Reproduce
JUnit-Test to reproduce: 

```
  @Test
  void testLiquibaseXmlBug() throws IOException {
    ByteArrayOutputStream stream = new ByteArrayOutputStream();
    DatabaseChangeLog log = new DatabaseChangeLog();
    ChangeLogSerializer serializer = new XMLChangeLogSerializer();
    ChangeSet set = new ChangeSet("Test", "test", Boolean.FALSE, Boolean.FALSE, null, "context", "oracle, mssql",log);
    List<ChangeLogChild> changes = new ArrayList<>();
    changes.add(set);
    serializer.write(changes, stream);
    assertEquals("<?xml version=\"1.1\" encoding=\"UTF-8\" standalone=\"no\"?>"
                 + "<databaseChangeLog xmlns=\"http://www.liquibase.org/xml/ns/dbchangelog\" xmlns:ext=\"http://www.liquibase.org/xml/ns/dbchangelog-ext\" xmlns:pro=\"http://www.liquibase.org/xml/ns/pro\" xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xsi:schemaLocation=\"http://www.liquibase.org/xml/ns/dbchangelog-ext http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-ext.xsd http://www.liquibase.org/xml/ns/pro http://www.liquibase.org/xml/ns/pro/liquibase-pro-4.1.xsd http://www.liquibase.org/xml/ns/dbchangelog http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-4.1.xsd\">"
                 + "    <changeSet author=\"test\" context=\"context\" dbms=\"oracle,mssql\" id=\"Test\" objectQuotingStrategy=\"LEGACY\"/>"
                 + "</databaseChangeLog>", new String(stream.toByteArray()));
  }
```

## Expected/Desired Behavior
Changelog should look like: 
 `<changeSet  dbms="oracle,mssql" ... >`


if (currentMd5Sum == null) {
            return true;
        }
        if (storedCheckSum == null) {
            return true;
        }
        if (currentMd5Sum.equals(storedCheckSum)) {
            return true;
        }
        for (CheckSum validCheckSum : validCheckSums) {
            if (currentMd5Sum.equals(validCheckSum) || storedCheckSum.equals(validCheckSum)) {
                return true;
            }
        }
        return false;
    }
    @Override
    public PreconditionContainer getPreconditions() {
        return preconditions;
    }
    @Override
    public void setPreconditions(PreconditionContainer preconditionContainer) {
        this.preconditions = preconditionContainer;
    }
    public void addSqlVisitor(SqlVisitor sqlVisitor) {
        sqlVisitors.add(sqlVisitor);
    }
    public List<SqlVisitor> getSqlVisitors() {
        return sqlVisitors;
    }