Move plainAndParameterizedHeaders to Method.

Before this was calculated inside of a controller which was masking
changes in the underlying model, preventing the display from being
updated.
Display parameters doesn't rely on cached values.

Before some of the data for this calculation was being cached and this
was preventing the display from being updated when the underlying model
changed.
Try it re-renders when a new method is selected.

Before, when a method was selected inside of the resource popover the
display would not update, showing the try it section for the originally
selected method.