U4-5097 - fixed issue with inital value of slider not being saved

http://issues.umbraco.org/issue/U4-5097 details how if a slider is added to a document type, and the content using that type is saved without the value being changed, the value is noted saved (it's missing from the umbraco.config) file.

This pull request adds a check for this and initialises the model value from the slider value in the UI if not already set.