user: fix double-unref of GDBusMethodInvocation throughout

When we return FALSE, we're not saying "failure," we're actually
saying "unhandled." So in accounts-user-generated.c (generated by
gdbus-codegen), _accounts_user_skeleton_handle_method_call() will
call g_dbus_method_invocation_return_error(), which assumes
ownership, sends a D-Bus error to the peer, and unrefs the
GDBusMethodInvocation. Problem is, we've already done all of that
and doing so twice is unexpected and bad.

Spotted by Ray Strode in !51.

Fixes #86
1 job for master in 51 seconds
Status Job ID Name Coverage
  Test
passed #1762514
build-fedora

00:00:51