title: How to read only unknown types as strings in Npgsql 3.1
body: <p>Using Npgsql 3.1.5 I try to load data from a table using <code>NpgsqlCommand.ExecuteReader()</code>. Some of the columns are custom Postgres types. I want to read those as text, like Npgsql 2.2 used to do. By default Npgsql 3.1 throws an error for those:</p>
  
  <pre><code>System.NotSupportedException : The field 'my_custom_type_field' has a type currently unknown to Npgsql (OID 50064). You can retrieve it as a string by marking it as unknown, please see the FAQ.
  </code></pre>
  
  <p>The <a href="http://www.npgsql.org/doc/faq.html" rel="nofollow">FAQ</a> suggests either setting <code>AllResultTypesAreUnknown</code> or specifying which columns are unknown, but neither of those is a good solution. <code>AllResultTypesAreUnknown</code> reads all columns as strings, which is useless to me. Specifying the individual column names one by one would take even more work than upgrading the code to use custom types properly, because there are many tables and many queries. I don't want to read <em>all</em> types as strings, I only want to read the <em>unknown</em> types as strings. In other words, read as strings when Npgsql would otherwise throw the above exception. Is there a way to get this behaviour?</p>