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`
