Cannot redefine parameters in subsequent version

[Gavin Panella]

Attempting to redefine parameters in a subsequent version fails:

    >>> @operation_parameters(foo=TextLine(title=u"2"))
    ... @operation_for_version(2)
    ... @operation_parameters(foo=TextLine(title=u"1"))
    ... @operation_for_version(1)
    ... def bar(foo): pass
    Traceback (most recent call last):
    ...
    TypeError: 'foo' parameter is already defined.

Perhaps the following check in operation_parameters.annotate_method()
could be removed:

    if name in params:
        raise TypeError(
            "'%s' parameter is already defined." % name)

?

(There's a work-around possible with operation_removed_in_version, but
that feels fairly hackish, and is certainly confusing the first time
someone sees it.)