Fixes EGI-FCTF/rOCCI-server#23, default format

Forcing default request format '*/*' to 'text/plain', as described
in the OCCI specification GFD.185 section 3.6.6.
Merge pull request #24 from arax/media_type_fix

Fixes EGI-FCTF/rOCCI-server#23, default format