@capture union problem

Similar to what was noted in https://github.com/FluxML/MacroTools.jl/issues/40#issuecomment-357466077,

```julia
julia> @capture :(myvariable = 2) (lhs_Symbol = rhs_)
true

julia> @capture :(myvariable = 2) (lhs_Symbol = rhs_) | someotherpat
false

julia> @capture :(myvariable = 2) (lhs_ = rhs_) | someotherpat
true
```

The second match should be true as well.

Start at the @capture entry point and reproduce the three Julia examples from the issue, comparing the union pattern with and without the typed lhs_Symbol binding. Done means the second example, `(lhs_Symbol = rhs_) | someotherpat`, matches the expression and returns true.

Written by the indexing model from the issue text.