Fix error in spec/support/matchers.rb

env.include?(…) will accept partial value of the exported environment
variable
Add two more spec examples around :env variables

New Aspects validated are:
 - correct management of quoted and unquoted definitions
 - the matcher validates the final value after 'export' call, and not
the variable definitinon (refered variables are evaluated).
Merge pull request #77 from gildegoma/fix-spec-env-matchers

Fix a weakness in 'should matcher' for environment varialbe and add 3 spec examples