Movie Scene Float sections do not blend when adding a key

Movie scene float tracks to not blend when adding new keys through the add key button if the value comes from outside sequencer. This is a result of a change introduced by CL: 4997300.  

1. Create a sequence and add a spotlight to it
2. Track the light's intensity, and add a key at frame 0 with a value of 10
3. Make the section infinite
4. Change the easing curve on intensity so that it blends for about half the sequence
5. Scrub to somewhere within the easing section
6. Edit the intensity through the light's details panel using the slider (don't enter the value)
7. Add a key by clicking the add key ([Image Removed] button) between the key navigation arrows

RESULT
The edited value is discarded, and the intensity is set to the sequence value (10).