Add libsoup to gir includes

Since the public API now references a type from libsoup, we need
to add Soup-2.4.gir to Rest_@API_VERSION_AM@_gir_INCLUDES.

https://bugzilla.gnome.org/show_bug.cgi?id=728340
More improvements to rest_proxy_call_get_params API doc
flickr: Fix function setting regression

Since commit c66b6d, RestProxyCall::url is regenerated after calling the
RestProxyCall::prepare virtual method. This breaks the flickr code as
it needs to reset RestProxyCall::url in order to remove the function
from it.
It used to do that by directly accessing RestProxyCall private
data. Since c66b6d, this can be solved using only public methods as
if the function is reset on the RestProxyCall in ::prepare, ::url will
be regenerated to reflect that after ::prepare and ::serialize_params
have been called.

https://bugzilla.gnome.org/show_bug.cgi?id=708359
lastfm: Fix function setting regression

Since commit c66b6d, RestProxyCall::url is regenerated after calling the
RestProxyCall::prepare virtual method. This breaks the lastfm code as
it needs to reset RestProxyCall::url in order to remove the function
from it.

It used to do that by directly accessing RestProxyCall private
data. Since c66b6d, this can be solved using only public methods as
if the function is reset on the RestProxyCall in ::prepare, ::url will
be regenerated to reflect that after ::prepare and ::serialize_params
have been called.

https://bugzilla.gnome.org/show_bug.cgi?id=708359
oauth: Make sure RestProxyCall::url is set before using it

The oauth "prepare" step needs RestProxyCall::url to be set, but since
commit c66b6d this is only set after the "prepare" and
"serialize_headers" methods have been called. As it's
better if OAuthProxyCall do not directly access RestProxyCall private
data, this commit adds a getter to RestProxyCall which will be used
to make sure RestProxyCall::url is set when OAuthProxyCall::prepare
needs it.

https://bugzilla.gnome.org/show_bug.cgi?id=708359
oauth: Remove direct access to RestProxyCallPrivate

OAuthProxyCall was directly accessing private RestProxyCall data. The
same functionality can be achieved using only librest public API, so
let's use that instead.

https://bugzilla.gnome.org/show_bug.cgi?id=735919
flickr: Remove direct access to RestProxyCallPrivate

Now that FlickrProxyCall no longer directly modifies RestProxyCall::url,
it's trivial to remove all accesses to RestProxyCallPrivate.

https://bugzilla.gnome.org/show_bug.cgi?id=735919
lastfm: Remove direct access to RestProxyCallPrivate

Now that LastFMProxyCall no longer directly modifies RestProxyCall::url,
it's trivial to remove all accesses to RestProxyCallPrivate.

https://bugzilla.gnome.org/show_bug.cgi?id=735919
Move RestProxyCallPrivate to rest-proxy-call.c

It was in a separate rest-proxy-call.h header which allowed derived
classes to access RestProxyCall private data without any control.
Better to go through explicit methods to do that.

https://bugzilla.gnome.org/show_bug.cgi?id=735919
Fix URL used for flickr rest calls

https must be used, and the correct URL for uploads is up.flickr.com

https://bugzilla.gnome.org/show_bug.cgi?id=735920
tests: Don't call g_type_init() on newer glib

This is deprecated and causes a compile warning

https://bugzilla.gnome.org/show_bug.cgi?id=735921
tests: Don't call soup_message_headers_get with newer libsoup

This is deprecated and causes compile-time warnings

https://bugzilla.gnome.org/show_bug.cgi?id=735921
tests: Remove unused 'session' variable

https://bugzilla.gnome.org/show_bug.cgi?id=735921
tests: Remove more unused variables/code

https://bugzilla.gnome.org/show_bug.cgi?id=735921
tests/oauth: Use oauthbin.com as the test server

The tests used to use term.ie which is gone.
There are still some intermittent failures when running the tests,
sometimes the test is successful, sometimes it's not :(

https://bugzilla.gnome.org/show_bug.cgi?id=735921
tests/lastfm: Fix memory leak

https://bugzilla.gnome.org/show_bug.cgi?id=735921
tests/proxy: Fix memory leaks

https://bugzilla.gnome.org/show_bug.cgi?id=735921
tests/proxy: Make sure GError is unset when call succeeds

https://bugzilla.gnome.org/show_bug.cgi?id=735921
oauth: Don't leak RestProxyCall::token{_secret}

https://bugzilla.gnome.org/show_bug.cgi?id=735922