Animation Editor : fix "multiple value" exception when dragging tangent
Animation Editor : fix "multiple value" exception when dragging tangent

ref #4756

GafferHQ/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,johnhaddon/gaffer