Bug 16776: Do not forget external language choice in language switcher

Bug 16776: Do not forget external language choice in language switcher

If language is set by external link language switcher does not work

To reproduce:

- Have a multilingual koha installation, let's say en, de-CH, fr-FR and it-IT
- Create external links e.g. from your multilingual library website to each language variant of OPAC:

...cgi-bin/koha/opac-main.pl?language=en
...cgi-bin/koha/opac-main.pl?language=de-CH
...cgi-bin/koha/opac-main.pl?language=fr-FR
...cgi-bin/koha/opac-main.pl?language=it-IT

Each link opens fine displaying the language as appropriate.

Now try to switch language on the page you just opened via the external link.

It won't work, it will always stay on the language defined by the external link.

To test:
- Apply patch
- Repeat steps above
- Verify that the language switcher works
- Verify in browser address field that after switching language, the param 'language= 'has vanished from url string
- Try links with multiple params and with params containing 'language', e.g.
  ...cgi-bin/koha/opac-search.pl?language=fr-FR&q=language&mylanguage=test
- Verify that only the parameter 'language=' vanishes

Tested counter patch, works as well.
Signed-off-by: Marc <veron@veron.ch>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>