fix db opening error with absolute path on windows

fix #282 (SQLITE_CANTOPEN error when cached.Database is called with absolute path on windows).
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Creating db using cached.Database with absolute path has error on windows.
below is my code and error message

code

    var sqlite = require('sqlite3');
    var current_db = new sqlite.cached.Database('current.db');    // OK
    var relative_db = new sqlite.cached.Database('folder\\current.db');    // OK
    var absolute_db = new sqlite.cached.Database('C:\\current.db');    // ERROR!!!

error message

    events.js:72
            throw er; // Unhandled 'error' event
                  ^
    Error: SQLITE_CANTOPEN: unable to open database file