fixed backslashes not getting escaped in field type json

added testJsonEscapeString

```
var json_dump = {
			Id	: id,
			data : {
				foo : 12,
				text : "first line\r\nsecond line\ttabbed"
			}
		}

INSERT INTO "JsonEscapeString" Values(
                    23456,
                    E'{"foo":12,"text":"first line\r\nsecond line\ttabbed"}'
                    )
```
this would break because it is not a valid json value in postgresql
http://stackoverflow.com/questions/935/string-literals-and-escape-characters-in-postgresql

https://github.com/theRemix/postgrehx/commit/b4c67bc6417fd0e879a0e8b2270a85e15139909f fixes this
```
INSERT INTO "JsonEscapeString" Values(
                    23456,
                    E'{"foo":12,"text":"first line\\r\\nsecond line\\ttabbed"}'
                    )
```