if baseParams has a FORMAT key, and the url contains FORMAT already, prevent that we have duplicate FORMAT parameters in the url, e.g. ArcGIS Server will not like this and will return an exception
implement @ahocevar's review/rewrite
Merge pull request #84 from bartvde/legendformat

if baseParams has a FORMAT key, and the url contains FORMAT already, pre... (r=@ahocevar)
Revert "Merge pull request #84 from bartvde/legendformat"

This reverts commit 244740b3a73c95907ef8700a84e57d9a84dc8da4, reversing
changes made to 289d47c53939a35033456fd8f4bc08c9e8c0215e.