start_account_report parameters param should be a dict rather than string

[cmurtaugh]

The inline doc for the start_account_report method indicates that the parameters argument should be a string, but it needs to be a dict. Also, it's not obvious what the keys of that dict need to be. After some experimentation, I found that the dict needs to look like:

{
    'parameters[enrollments]': '1',
    'parameters[include_deleted]': '1',
}

This is a little clunky looking to me and non-obvious; we could improve the situation by providing more details in the documentation, or we could translate the dict keys to the format expected by the Canvas API (parameters[<k>]).

What does everyone think about automatically translating the paramaters dict keys vs. just providing better docs?

[rascalking]

Personally, I'd love to sidestep the issue entirely and tweak the codegen to output methods that post via JSON rather than form fields.

[elliottyates]

@rascalking on your todo list? :)