GRDB.swift: DatabasePool can't read from existing non-WAL database

I ran into the same issue as #40 and did a little digging. I appears that a DatabasePool connection fails if no SerializedDatabase exists for the connection yet. I wrote a sample which illustrates this below.

` import UIKit import GRDB

class ViewController: UIViewController {

internal static var connectionPool = [String:DatabasePool]()

override func viewDidLoad() {
    super.viewDidLoad()
    doStuff()
}

func doStuff()
{
    //a call to this db creates the .db-shm and .db-wal files
    createTable("WillNotFail")
    getAllTables("WillNotFail")

    //this call does not and throws an exception
    getAllTables("WillFail")
}

internal func getPooledConnection(name:String) -> DatabasePool?
{
    do
    {
        if ViewController.connectionPool[name] == nil
        {
            let rootLibDirectory = NSSearchPathForDirectoriesInDomains( .LibraryDirectory, .UserDomainMask, true ).first as String!
            print(rootLibDirectory)

            ViewController.connectionPool[name] = try DatabasePool( path: "\(rootLibDirectory)/\(name).db" )
        }
    }
    catch
    {
        print("\(error)")
        return nil
    }

    return ViewController.connectionPool[name]
}

internal func createTable(name: String)
{
    guard let pool = getPooledConnection(name)
        else
    {
        return
    }

    do
    {
        try pool.write
        { db in
            try db.execute("CREATE TABLE IF NOT EXISTS SampleTable (col1 varchar)")
        }
    }
    catch
    {
        print(error)
    }

    return
}

internal func getAllTables(name: String)
{
    guard let pool = getPooledConnection(name)
    else
    {
        return
    }

    pool.read
    { db in

        let results = Row.fetchAll( db, "SELECT name FROM sqlite_master WHERE type = 'table' ORDER BY name")

        for result in results
        {
            print( result.value(atIndex: 0) )
        }
    }

    return
}

}

`