Create Language Annotation for parsed "Content-Language" header

Stanbol supports parsing the language of the content by using the "Content-Language" header since STANBOL-660. However currently only the `dc:language` property is set for the ContentItem.

However based on the specification of STANBOL-613 this information is only used as fallback if no language annotation is present in the ContentItem. So as soon as any Language Identification Engine is present in the Chain the "Content-Language" as parsed by the User will get ignored. This is not the intention of a user explicitly parsing the language.

To force Stanbol to use the parsed language a Language Annotation with the confidence 1.0 needs to be added to the metadata of the ContentItem instead.