Use inout for ref returning members in std.container.array

For motivation see 

http://forum.dlang.org/thread/agyeejbrpotpjrecepyl@forum.dlang.org#post-wysehvbsfnezfozsagpl:40forum.dlang.org
Don't use inout when inout is not also present in the return type (or an out parameter). If you'd done so, you'd notice the problem I mentioned in a line comment on the unit test.