Added hanlding of valid NSData object with 0 bytes for serializers
Extracted default HTTP header creation into a public class method.
Make extension friendly
Merge pull request #273 from technomage/master

Make extension friendly
Merge pull request #261 from cnoon/master

Extracted default HTTP header creation into a public class method
[Issue #269] Fixing crash caused by Manager deinit
[Issue #269] Documenting requirement to cancel pending tasks before session manager deinitialization
Merge pull request #254 from mowens/bug/handle-0-bytes

Added hanlding of valid NSData object with 0 bytes for serializers