fix minitwit/flaskr test errors, improve docs about file open mode

app.open_resource needs to get called with the correct mode param (python3
will read bytes [not str] if the wrong mode is used), add mode param docs.

rv.data is bytes, fix the data type we compare it with to be also bytes

flask/app.py

@@ -630,6 +630,7 @@ class Flask(_PackageBoundObject):
 
         :param resource: the name of the resource.  To access resources within
                          subfolders use forward slashes as separator.
+        :param mode: resource file opening mode, default is 'rb'.
         """
         return open(os.path.join(self.instance_path, resource), mode)
 

flask/helpers.py

@@ -838,6 +838,7 @@ class _PackageBoundObject(object):
 
         :param resource: the name of the resource.  To access resources within
                          subfolders use forward slashes as separator.
+        :param mode: resource file opening mode, default is 'rb'.
         """
         if mode not in ('r', 'rb'):
             raise ValueError('Resources can only be opened for reading')