soup-uri: minor code reorg/renaming

Change uri_decoded_copy() to strdup the input string *after* doing its
g_return_val_if_fail(), to avoid pointlessly confusing code analyzers.

Also, rename the function to soup_uri_decoded_copy(), since it's used
from soup-request-data.c too and it's been annoying me that it was
non-namespaced.

https://bugzilla.gnome.org/show_bug.cgi?id=709793