If optimizer runs several times it can try to push filters into
table provider several times. But, `supports_filters_pushdown`
may be context-dependent. For example, it may support any filter
like `column=value` but not the both at the same time.

Consider the following optimizer run:

1. Try to push `a = 1, b = 1`.
   `supports_filters_pushdown` returns [Exact, Inexact]
   Ok, will remember that `a=1` is pushed and make a filter node for `b=1`.

...
Another optimization iteration.

2. Try to push `b = 1`.
   `supports_filters_pushdown` returns [Exact]. Of course, the
   table provider can't remember all pushed filters. So, there is nothing
   to left but to answer: "Exact".
   Now, the optimizer thinks that conjunction `a = 1 AND b = 1` is supported
   exactly, but it's not.

To prevent this problem, this patch adds already pushed filters to the
`supports_filters_pushdown` call.