Add fetch body types for $ajax method

[Roronoro]

Sorry for the delay.

Here is my suggestion for letting $ajax method accept the same body types as fetch:

• In the parseFormData() function I added more ifs returning types fetch supports.
• When building the request object the enctype is set to null if not explicit set otherwise.
• If enctype is null it is later deleted from the request object. This lets fetch itself decide the enctype.
• I've added 3 test cases for the Ajax method.

issue #142