Fix for GetRenderingParameters

Commit 0c789988a70f8f5b5153990a2d0efaabfd5de7c2 updated ```GetRenderingParameters(NameValueCollection collection)``` to perform a null-check on the SitecoreTypeConfiguration, and load it if it was null.

However, the remaining method that took the parameters as a string bypassed this. This commit has changed this to go via the updated method. This also is consistent with the behaviour of the other 2 ```GetRenderingParameters``` methods.