Fix bearer token retrieval, device operation content type and initialisation

[rconnett]

No description provided.

[RafaRuiz]

• Does this require a change in the README?
• Any breaking change? as the constants have been used
• Could you please explain briefly why the change between localStorage.token to localStorage[AUTH_TOKEN]
  (or mainly why the change between localStorage.foo -> localStorage[BAR])

[rconnett]

You shouldn't need to change the RADME.
I don't see a breaking change however Marwan indicated that it would cause problems for one of your clients so I've just been using our forked version since then.
The updates to the localStorage usage was to fix what was already there. Some of the code had been updated to use the constants for localStorage previously but the code I've updated had been left as not using the constants so those bits it just didn't work.