videorate: negotiation failure with input rate 0/1 drop-only=true and downstream capsfilter

Submitted by Tim Müller `@tpm`

Only happens with drop-only=true.

Reproduce with:

$ gst-launch-1.0 videotestsrc num-buffers=50 ! vp8enc ! matroskamux ! filesink location=/tmp/foo.mkv

$ gst-launch-1.0 filesrc location=/tmp/foo.mkv ! decodebin ! videorate drop-only=false ! video/x-raw,framerate=1/5 ! fakesink

--> works

$ gst-launch-1.0 filesrc location=/tmp/foo.mkv ! decodebin ! videorate drop-only=true ! video/x-raw,framerate=1/5 ! fakesink

--> not-negotiated error

(Let's ignore for now the fact that matroskamux/matroskdemux encode a rate of 0/1 here.)