Allow HttpContent body/return types for extra customizability.

[clairernovotny]

This should fix #78

[bennor]

We're clearly still missing the R# setting that stops it from indenting chained method calls like this. Would be good to clean that up so we don't get unnecessary changes like this. I wonder if there's a way to disable R# entirely in test case files, because as you can see below, its "clean up" feature has busted some of the tests.

[bennor]

Looking at this on my machine, are you sure your local settings aren't somehow overriding the project settings? These indents shouldn't be happening - if I do Tools > Cleanup Code... then select Default: Reformat Code, these indents are reset to what they were before.

[bennor]

Do you reckon this will make #83 is unnecessary? With a few small changes to this PR, we could support any type that derives from HttpContent instead of just HttpContent itself, then custom serialization of any type could be achieved with a class that derives from HttpContent.

Would mean we wouldn't have to sacrifice readability/usability on the interfaces, because it would allow something like this:

public interface IWhateverApi
{
    [Get("/some-xml-endpoint")]
    XmlContent GetSomeXml();
}

[anaisbetts]

@onovotny I think if you burn the "Fix formatting" commit, this might be salvageable