Strings containing quotes are not properly quoted by the GTFSWriter

[thzinc]

Any string value that is written using the GTFSWriter.WriteFieldString(string, string, string) signature where the value contains a " but no , is not properly escaped in the result.

Example call:

this.WriteFieldString("trips", "trip_headsign", "\"this is a string containing a quoted value\"");

Expected:

"""this is a string containing a quoted value"""

Actual:

""this is a string containing a quoted value""

[thzinc]

Also found that reading GTFS files with strings containing escaped quotes failed too