Added option to override content type used for json updates.

Certain services requires specific settings for this content type,
e.g Microsoft Service Provider foundation requires it to be set to Application/json;odata=verbose
Edm.Int64 should be string formatted according to odata json spec
Fix formatting of collections for json output
Fix handling exceptions that are not http exceptions
Fix parsing of null strings. These got parsed as empty strings
Added specs to verify json serialization of objects
Merge pull request #29 from sigmunau/develop

A few small changes that are needed to interact with the Microsoft Service Provider Foundation OData Services