[CVS ci] invalidate method cache in store_global

[Leopold Toetsch]

If a name is stored into a namespace that starts with two underscores, 
the method cache is invalidated now.

The follwing program runs now as expected:
.sub main @MAIN
    .local pmc o, cl
    newclass cl, "Foo"
    subclass cl, cl, "Bar"
    $I0 = typeof cl
    o = new $I0
    print o
    $P0 = global "ok2"
    store_global "Bar", "__get_string", $P0
    print o
.end
.sub ok2
    .return("ok 2\n")
.end
.namespace [ "Foo" ]
.sub __get_string
    .return("ok 1\n")
.end
leo