Merge 3 minor fixes from forcetk library (in developerforce) to entity framework's forcetk

Merge 3 minor fixes from forcetk library (in developerforce) to entity framework's forcetk.

1. Fix retrieve method's logic
2. Update version to v27.0
3. Dont send contentType as JSON for GET or DELETE http requests as some libraries like in Node.js Express BodyParser middleware break.