- When attempting to remove a created lambda permission with the new fix from https://github.com/localstack/localstack/issues/1260, throws up `SerializationError`

Code being called:

```
func deleteLambdaPermission(ctx context.Context, destinationLambdaARN, permissionStatementID string) (err error) {
	lambdaClient := services.LambdaClient()
	lambdaRemovePermissionInput := &lambda.RemovePermissionInput{
		FunctionName: aws.String(destinationLambdaARN),
		StatementId:  aws.String(permissionStatementID),
	}
	_, err = lambdaClient.RemovePermissionWithContext(ctx, lambdaRemovePermissionInput)

	return
}
```

Checking for error in tests shows the following

```
 Expected
        <*awserr.requestError | 0xc0004a2180>: {
            awsError: {
                code: "SerializationError",
                message: "failed to unmarshal response error",
                errs: [
                    {
                        awsError: {
                            code: "UnmarshalError",
                            message: "failed decoding error message",
                            errs: [
                                {
                                    msg: "invalid character '<' looking for beginning of value",
                                    Offset: 1,
                                },
                            ],
                        },
                        bytes: [60, 33, 68, 79, 67, 84, 89, 80, 69, 32, 72, 84, 77, 76, 32, 80, 85, 66, 76, 73, 67, 32, 34, 45, 47, 47, 87, 51, 67, 47, 47, 68, 84, 68, 32, 72, 84, 77, 76, 32, 51, 46, 50, 32, 70, 105, 110, 97, 108, 47, 47, 69, 78, 34, 62, 10, 60, 116, 105, 116, 108, 101, 62, 52, 48, 52, 32, 78, 111, 116, 32, 70, 111, 117, 110, 100, 60, 47, 116, 105, 116, 108, 101, 62, 10, 60, 104, 49, 62, 78, 111, 116, 32, 70, 111, 117, 110, 100, 60, 47, 104, 49, 62, 10, 60, 112, 62, 84, 104, 101, 32, 114, 101, 113, 117, 101, 115, 116, 101, 100, 32, 85, 82, 76, 32, 119, 97, 115, 32, 110, 111, 116, 32, 102, 111, 117, 110, 100, 32, 111, 110, 32, 116, 104, 101, 32, 115, 101, 114, 118, 101, 114, 46, 32, 73, 102, 32, 121, 111, 117, 32, 101, 110, 116, 101, 114, 101, 100, 32, 116, 104, 101, 32, 85, 82, 76, 32, 109, 97, 110, 117, 97, 108, 108, 121, 32, 112, 108, 101, 97, 115, 101, 32, 99, 104, 101, 99, 107, 32, 121, 111, 117, 114, 32, 115, 112, 101, 108, 108, 105, 110, 103, 32, 97, 110, 100, 32, 116, 114, 121, 32, 97, 103, 97, 105, 110, 46, 60, 47, 112, 62, 10],
                    },
                ],
            },
            statusCode: 404,
            requestID: "",
            bytes: nil,
        }
    to be nil
```
response_obj = jsonify(response_obj)
        else:
            response_obj = str(response_obj)
            details['Headers']['Content-Type'] = 'text/plain'
        return response_obj, details['StatusCode'], details['Headers']
    
    not_found = None
    if arn not in arn_to_lambda:
        not_found = not_found_error(arn)
    elif qualifier and not arn_to_lambda.get(arn).qualifier_exists(qualifier):
        not_found = not_found_error('{0}:{1}'.format(arn, qualifier))
    if not_found:
        forward_result = forward_to_fallback_url(func_arn, data)
        if forward_result is not None:
            return _create_response(forward_result)
        return not_found
    if invocation_type == 'RequestResponse':
        result = run_lambda(asynchronous=False, func_arn=arn, event=data, context={}, version=qualifier)
        return _create_response(result)
    elif invocation_type == 'Event':
        run_lambda(asynchronous=True, func_arn=arn, event=data, context={}, version=qualifier)
        return _create_response('', status_code=202)
    elif invocation_type == 'DryRun':
        
        return _create_response('', status_code=204)
    return error_response('Invocation type not one of: RequestResponse, Event or DryRun',
                          code=400, error_type='InvalidParameterValueException')
@app.route('%s/event-source-mappings/' % PATH_ROOT, methods=['GET'])
def list_event_source_mappings():