Fix SOQL single quote escape error

The existed escaping in backend.query.quoted_string_literal does not produce valid SOQL.

Using the existing escaping raises a SalesforceError (unexpected token)

This PR changes the replacement from ```''``` to ```'\```as recommended in the [SalesForce SOQL documentation](http://www.salesforce.com/us/developer/docs/soql_sosl/Content/sforce_api_calls_soql_select_quotedstringescapes.htm).

I've added tests for queries generated using .filter as well as .raw.