Consider checking for security context in the PresentationRequest constructor #362

@mounirlamouri brought up the following:

Currently, we check for mixed content in the steps to execute start(), reconnect() and getAvailability(). It would simplify both the spec and the implementation to move this check to the PresentationRequest constructor.

I am not sure if it is okay to throw exceptions in conctructors of Web APIs. If there are precedents, I'd like to know.

Also, we would need to be assured that the security context could not be changed between constructor and method invocation.