Reassigning Variables within @capture

`@capture` introduces assignments to variables indicated in its second argument. When one of these assignments is to a variable with the same name as the first argument, things seem to go wrong — the match fails and the value of that variable is set to `nothing`. I think the better behavior in this case would be for the match to succeed, and for `ex` and `b` in the second example to have the values of `a` and `b` in the first.

```
julia> ex = :(1 + 2)
:(1 + 2)

julia> @capture(ex, a_ + b_)
true

julia> @capture(ex, ex_ + b_)
false

julia> ex
```