bug / lack of QuoteIdent

Hi,

There is a minor bug in the `String` method here:

https://github.com/influxdata/influxql/blob/b9e521f450818862cab2bc31a30293b192439728/ast.go#L2602

Should be:
```
if s.Database != "" {
_, _ = buf.WriteString(" ON ")
_, _ = buf.WriteString(QuoteIdent(s.Database))
}
```
See:
https://github.com/influxdata/influxql/blob/master/ast.go#L2533

Open ast.go at the String method around the referenced line and compare it with the master version around line 2533. Confirm that the database name is written through QuoteIdent, then verify that the generated ON clause uses the quoted identifier.

Written by the indexing model from the issue text.