overview map uses projection parameter or base projection before defaults

Addresses #2998:
- creates a view object using options.projection if specified, default EPSG:3857 otherwise
- if a map view exists upon adding the control to a map, the default is overriden with the map view projection

(shouldn't a system default projection be defined somewhere in ol.proj.Projection.defaults or the like?)
I found it less intuitive since (currently) all other View parameters are being overridden by the control during use. It could be a variant if the View actually has more parameters that influence its behaviour, maybe even as an alternate parameter. What do you think?