quick fix for GeoJSON adapter

On some installations I run into the following error when trying to load a GeoJSON file:
```
$jsongeom = geoPHP::load(file_get_contents($this->geojsonFileName),'json');
```
The file is named test.json.

error/stacktrace:
```
[error] PHP Fatal error:  Uncaught exception 'Exception' with message 'geoPHP could not find an adapter of type geojson' in /dokuwiki/lib/plugins/geophp/geoPHP/geoPHP.inc:71
Stack trace:
#0 /dokuwiki/lib/plugins/openlayersmap/StaticMap.php(567): geoPHP::load('{?"type": "Feat...', 'geojson')
#1 /dokuwiki/lib/plugins/openlayersmap/StaticMap.php(597): StaticMap->autoZoom()
#2 /dokuwiki/lib/plugins/openlayersmap/helper/staticmap.php(99): StaticMap->getMap()
#3 /dokuwiki/lib/plugins/openlayersmap/syntax/olmap.php(565): helper_plugin_openlayersmap_staticmap->getMap('52.08', '5.19', '11', '600x600', NULL, Array, '', '', 'samples:test.js...')
#4 /dokuwiki/lib/plugins/openlayersmap/syntax/olmap.php(116): syntax_plugin_openlayersmap_olmap->_getStaticOSM(Array, Array)
#5 /dokuwiki/inc/parser/handler.php(75): syntax_plugin_openlayersmap_olmap->handle('<olmap id="olMa...', 5, 680, Object(Doku_Handler))
#6 /dokuwiki/inc/parser/lexer.php(517): Doku_Handler->plugin('<olmap id="olMa...', 5, 680, 'openlayersmap_o...')
#7 /dokuwiki/inc/parser/lexer.php( in /dokuwiki/lib/plugins/geophp/geoPHP/geoPHP.inc on line 71
```
It seems there is some autolookup going on ignoring the 'json' parameter.