Treat string options as case sensitive

Without this modification every String option is lowercased.
At least for path fragments, this behaviour is clearly wrong.
e.g.:
``coap://locahost/states/CHryD``
would be interpreted as
``coap://locahost/states/chryd``

[See section 6.4 of the coap-08 draft](http://tools.ietf.org/html/draft-ietf-core-coap-08#section-6.4)