Include body.class feedback for non-String body error

[aploshay]

A VCR::Request object performs this following initialization check:

        if body && !body.is_a?(String)
          raise ArgumentError, "#{self.class} initialized with an invalid body: #{body.inspect}."
        end

The error is raised for a non-String body, so it's helpful for the error message to return the actual body.class -- but body.inspect isn't a robust way to provide that, since some but not all objects include the class in their .inspect output. So this PR explicitly includes body.class output.

It also parenthetically mentions "(non-String)" after "invalid", to try to convey the issue is about the class check and not anything with the content of the body, itself.

Resolves #748