Fix MySQL support in ERXExistsQualifier

The identifier quote character in MySLQ is the backtick (not double quote). Using double quote resulted in invalid expression. Also, by using expression.sqlStringForSchemaObjectName we respect com.webobjects.jdbcadaptor.MySQLExpression.enableIdentifierQuoting flag which is off by default in case of MySQL (resulting in no quoting at all).
Merge branch 'master' of git://github.com/wocommunity/wonder

Conflicts:
	Frameworks/Core/ERExtensions/Sources/er/extensions/eof/qualifiers/ERXExistsQualifier.java