Tests fail on Redmine 3.1

[thorsten-de]

config_test fails on Redmine 3.1 (as seen in my PR)

rake test TEST=plugins/redmine_amazon_s3/test/unit/config_test.rb
/var/lib/gems/2.3.0/gems/htmlentities-4.3.1/lib/htmlentities/mappings/expanded.rb:465: warning: key "inodot" is duplicated and overwritten on line 466
Run options: --seed 6051

Running:

.E.E.EE...

Finished in 0.008554s, 1169.0793 runs/s, 2688.8823 assertions/s.

1. Error:
   ConfigurationTest#test_region_getter:
   NoMethodError: undefined method `split' for nil:NilClass

2. Error:
   ConfigurationTest#test_access_key_id_getter:
   NoMethodError: undefined method `split' for nil:NilClass

3. Error:
   ConfigurationTest#test_secret_access_key_getter:
   NoMethodError: undefined method `split' for nil:NilClass

4. Error:
   ConfigurationTest#test_bucket_getter:
   NoMethodError: undefined method `split' for nil:NilClass

10 runs, 23 assertions, 0 failures, 4 errors, 0 skips