Currently soup_form_request_for_data() seems to leak the uri.  This patch fixes
the leak.

Valgrind log:
==24835== 108 (64 direct, 44 indirect) bytes in 1 blocks are definitely lost in loss record 1,333 of 1,476
==24835==    at 0x4C221A7: malloc (vg_replace_malloc.c:195)
==24835==    by 0x612B504: g_malloc (gmem.c:132)
==24835==    by 0x6140409: g_slice_alloc (gslice.c:836)
==24835==    by 0x6140715: g_slice_alloc0 (gslice.c:845)
==24835==    by 0x5075021: soup_uri_new_with_base (soup-uri.c:177)
==24835==    by 0x5075AE1: soup_uri_new (soup-uri.c:401)
==24835==    by 0x505D6B7: soup_form_request_for_data (soup-form.c:357)
==24835==    by 0x4E2DFB5: prepare_message (rest-proxy-call.c:684)
==24835==    by 0x4E2E15C: rest_proxy_call_async (rest-proxy-call.c:737)