From a basic functionality standpoint, this accomplishes the same purpose as
the previous implementation using `_.omit`. However, rather than removing the
`selected` attribute from the sources, this will keep the structure the same by
only altering the value of the source.
If your usage of this plugin requires a stable source format, e.g. for
comparing sources to see if the source changed, please provide the `selected`
attribute when programmatically setting the player source. At this time, we
don't have a simple way to standardize the `selected` attribute on initial
source load. (i.e. It's up to the programmer to set the initial format on the
sources before giving them to the player)
As things turn out, the middleware constructor is called every time `setSource`
is called [1]. This was causing a new listener to get bound to
`QUALITY_SELECTED` on each change by the quality selector. :( This change makes
it so the listener is only bound on the initial creation of the player.
[1] 03529163b6/src/js/tech/middleware.js (L66)
