Navigate method: Only set explicitNavigation to true when trigger is true

See discussion: https://groups.google.com/forum/#!topic/durandaljs/PwYC9Apzc5Q
Missed checking if options isObject before attempting to access property
Missed checking if options is undefined (which defaults trigger: true)
Merge branch 'patch-1' of https://github.com/AdamWillden/Durandal into Version-2.1.0
Fix Issue #409: Navigate method: Only set explicitNavigation to true when trigger is true