Fix GeoServer format option param key

See http://docs.geoserver.org/latest/en/user/services/wms/vendor.html#format-options
Move serverType option to olx.source.ImageWMSOptions
Export ol.source.wms.ServerType.*
Cast serverType option in examples
Merge pull request #1412 from fredj/vector-api-hidpi-cleanup

[vector-api] hidpi cleanup