Add support for the user preferences endpoints (/api/0.6/user/preferences)

[metaodi]

The user preferences API is not implemented in osmapi and is not covered by #156, which is about user/details (and, per its comments, the messaging API and user blocks).

Preferences are arbitrary key/value pairs stored per user, intended for editors and tools to persist their own settings server-side.

Endpoints

From config/routes.rb:

resources :user_preferences, :except => [:new, :create, :edit], :param => :preference_key, :path => "user/preferences" do
  collection do
    put "" => "user_preferences#update_all", :as => ""
  end
end

which gives:

Method	Path	Purpose
GET	/api/0.6/user/preferences	all preferences of the authenticated user
GET	/api/0.6/user/preferences/:key	a single value (plain text response)
PUT	/api/0.6/user/preferences/:key	set a single value (plain text request body)
PUT	/api/0.6/user/preferences	replace all preferences with the posted document
DELETE	/api/0.6/user/preferences/:key	delete a single preference

The list response is a flat map:

<osm>
  <preferences>
    <preference k="somekey" v="somevalue"/>
  </preferences>
</osm>

Things to decide

• The single-key endpoints are not XML. GET .../preferences/:key returns the bare value as text/plain, and PUT .../preferences/:key takes the bare value as the request body. That is the first place in osmapi where a response is not parsed as XML and a request body is not built by xmlbuilder._xml_build, so http.OsmApiSession and the mixin need to handle a plain-text payload. Worth checking _put's return_value handling here.
• PUT /user/preferences replaces everything, it does not merge — deleting any key not present in the document. The docstring must be explicit about that, it is an easy way for a caller to wipe another tool's settings.
• All of these require authentication (before_action :authorize), so they are auth_api tests.

Suggested naming, following the existing convention:

api.user_preferences_get()            # -> {"key": "value", ...}
api.user_preference_get(key)          # -> "value"
api.user_preference_update(key, value)
api.user_preferences_update_all(prefs)
api.user_preference_delete(key)