emms-seek-* function does not match docstring or manual

While working on something else, I discover that the functionality of
emms-seek and emms-seek-to do not match their documentation (doc-string or
manual).

The doc-string for emms-seek-* says:

  "...
TIMESTAMP can be:

- A single number, in which case it is interpreted as seconds.

- A string of form [HH:]MM:SS.m, where HH is hours, MM is
  minutes, and SS is seconds.

In both forms seconds can be a floating point number."

However (emms-seek 20.0), for example, raises an error:

Wrong type argument: stringp, 20.0

The reason is that TIMESTAMP is fed to emms-timespec-to-secs which expects
a string.

---Fran