bug / lack of QuoteIdent

[antonijakubiak-samsung]

Hi,

There is a minor bug in the String method here:

influxql/ast.go

Line 2602 in b9e521f

_, _ = buf.WriteString(s.Database)

Should be:

	if s.Database != "" {
		_, _ = buf.WriteString(" ON ")
		_, _ = buf.WriteString(QuoteIdent(s.Database))
	}

See:
https://github.com/influxdata/influxql/blob/master/ast.go#L2533