Remove String requirement for 'data' parameter on Http shortcut methods

[xose]

The Http.call() method does not have any restriction on the type of the 'data' parameter. This allows interceptors to serialize any data type into any format (by default JSON). But the shortcut methods (post(), put(), ...) impose a String restriction, forcing serialization to be done before calling the method.

[jbdeboer]

@markovuksanovic You have been working with Http recently; could you take a look at this one?