Take coerce_to_string into account when handling DecimalField

Closes #62.

docs/changelog.rst

@@ -2,6 +2,13 @@
 Changelog
 #########
 
+*********
+**1.5.0**
+*********
+
+- **FIXED:** the ``coerce_to_string`` is now respected when setting the type, default value and min/max values of
+  ``DecimalField`` in the OpenAPI schema (:issue:`62`)
+
 *********
 **1.4.0**
 *********