Fix a problem with camelCase key and default value

The default was never passed on after parsing the key.
Extract module for prefixing a string

The same logic was already in use in two places and a third one is on
the way...
Fix a problem with partially overwritten defaults

While fixing issue #15 this case was overlooked. Only ran into it
when actually trying to use real environment variables to configure
the software...
Merge pull request #19 from ehuisman/master

Fix a problem with camelCase key and default value and with partially overwritten defaults