enabler(backends): allow ssl string parameters in PostgreSQL URL (#575)

The underlying library asyncpg accepts string values in the ssl
parameter. The old code only accepted the values true and false, which
are converted to boolean.

databases/backends/postgres.py

@@ -55,7 +55,11 @@ def _get_connection_kwargs(self) -> dict:
         if max_size is not None:
             kwargs["max_size"] = int(max_size)
         if ssl is not None:
-            kwargs["ssl"] = {"true": True, "false": False}[ssl.lower()]
+            ssl_lower = ssl.lower()
+            if ssl_lower == "true":
+                kwargs["ssl"] = True
+            elif ssl_lower == "false":
+                kwargs["ssl"] = False
 
         kwargs.update(self._options)