Custom HTTPException should support specific error handlers by type

[RobertoPrevato]

Custom HTTPException currently support specific exception handlers only by their HTTP status code. It is best supporting specific exception handlers by type, and give them precedence.

That is, in baseapp.pyx:

    cdef object get_http_exception_handler(self, HTTPException http_exception):
        try:
            return self.exceptions_handlers[type(http_exception)]
        except KeyError:
            return self.exceptions_handlers.get(http_exception.status, common_http_exception_handler)