text: Title: It fails when use all severity types in "--allure-severities" option
  Body: [//]: # (
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  #### I'm submitting a ... 
    - [ ] bug report
    - [ ] feature request
    - [ ] support request => Please do not submit support request here, see note at the top of this template.
  
  #### What is the current behavior?
  
  
  #### If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem
  
  
  #### What is the expected behavior?
  
  
  #### What is the motivation / use case for changing the behavior?
  
  
  #### Please tell us about your environment:
  
  - Allure version:        2.1.0
  - Test framework:        pytest@3.0
  - Allure adaptor:        allure-pytest@2.0.0b1
  
  #### Other information 
  
  [//]: # (
  . e.g. detailed explanation, stacktraces, related issues, suggestions 
  . how to fix, links for us to have more context, eg. Stackoverflow, Gitter etc
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