Fix wrong usage of `isinstance`

Second parameter should be a class, not string

Change-Id: I6401429d7cc314502a364b7fc81e73d30175b193
Signed-off-by: Anderson Bravalheri <a.bravalheri@bristol.ac.uk>
diff --git a/osm_ro/db_base.py b/osm_ro/db_base.py
index e7a4fb3..a4cdef3 100644
--- a/osm_ro/db_base.py
+++ b/osm_ro/db_base.py
@@ -170,7 +170,7 @@
         return self.con.escape(value)
 
     def escape_string(self, value):
-        if isinstance(value, "unicode"):
+        if isinstance(value, unicode):
             value = value.encode("utf8")
         return self.con.escape_string(value)