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 (merged).

Fixes #86 (closed)