Only add the ga: prefix if it is required
This checks if ga keys are already prefixed with ga: and only adds the
prefix if required. The previous behaviour was particularly anoying for
complex filters where keys within the filter would need a ga: prefix but
the first one could not have the prefix.

eg. Both the following would fail:
`ga:pagepath=/foo;ga:city=London`
`pagepath=/foo;city=London`

Now the following works
`ga:pagepath=/foo;ga:city=London`

needs a test, however I don't think the `ga` infrastructure is currently tested on ci at all though. 
 closing as stale, but if you would like to reopen and fixup pls do.
