Ignore None values per OAuth spec where empty values should be omitted
ignore pycharm project folder
Actually we shouldn't ignore, we should send an empty value not the
string None . .. per

http://oauth.net/core/1.0/#anchor14

Parameters are concatenated in their sorted order into a single string.
For each parameter, the name is separated from the corresponding value
by an ‘=’ character (ASCII code 61), even if the value is empty. Each
name-value pair is separated by an ‘&’ character (ASCII code 38)
Merge pull request #1 from KAPx/ignore-none-values

Set None values to an empty string so that this library doesn't turn them into 'None'