Allow pid repos in Ecto.Adapters.SQL.table_exists? (#729)

Closes #728.

Author: simoncse

lib/ecto/adapters/sql.ex

@@ -710,7 +710,7 @@ defmodule Ecto.Adapters.SQL do
   The table is checked against the current database/schema in the connection.
   """
   @spec table_exists?(Ecto.Repo.t(), table :: String.t(), opts :: Keyword.t()) :: boolean
-  def table_exists?(repo, table, opts \\ []) when is_atom(repo) do
+  def table_exists?(repo, table, opts \\ []) when is_atom(repo) or is_pid(repo) do
     %{sql: sql} = adapter_meta = Ecto.Adapter.lookup_meta(repo)
     {query, params} = sql.table_exists_query(table)
     query!(adapter_meta, query, params, opts).num_rows != 0