Fix parsing non-animation string as animation.
Fix parsing non-animation string as animation.

Previously, a string containing only a value - with no equal sign to
assign it to a keyframe position - would get parsed into a single,
discrete keyframe at 0. This could pollute the construction of an
animation using APIs by including a superfluous keyframe.

j-b-m/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,j-b-m/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,mltframework/mlt,j-b-m/mlt,j-b-m/mlt