Fix typo in VimRenderer.render

segment_info should have been initialized with self.segment_info, not updated: 
new values must have priority. Refactored the result so that static values are 
defined in VimRenderer.segment_info.
Add tests
Merge pull request #896 from ZyX-I/fix-vim-environment

Fix typo in VimRenderer.render

Ref jmcantrell/vim-virtualenv#26.