only one child element will be processed of which a `tei:lem` will get precedence

fixes https://github.com/baz-ga/bazga-webapp/issues/31

add tests for date:rfc822 and improve compliance, closes #6

brought up in #2

brought up in #2 ; the $get-language-string callback now reveives a parameter with the current language code.

since it's not available within the eXistdb build artifact anymore

instead of the hard coded namespace prefix

and update functions accordingly

where some calls to `wega-util-shared:guess-mimeType-from-suffix()` with parameter 'xml' returned the empty sequence