Fix mapTransactionIsolationLevel() for JRuby 9.2
Fix mapTransactionIsolationLevel() for JRuby 9.2

In JRuby 9.2 RubySymbol.toString() was changed in 4a0746 and no longer
returns an interned String. Use RubySymbol.asJavaString() instead.

jruby/activerecord-jdbc-adapter,kares/activerecord-jdbc-adapter,bruceadams/activerecord-jdbc-adapter,jruby/activerecord-jdbc-adapter,bruceadams/activerecord-jdbc-adapter,kares/activerecord-jdbc-adapter,kares/activerecord-jdbc-adapter,bruceadams/activerecord-jdbc-adapter,jruby/activerecord-jdbc-adapter