type/default: double escape colon in legend/filename when not using canvas

Since commit v0.3-97-g5793a8c [integrate jsrrdgraph in CGP], double quotes have
been removed around filenames and legend because it shouldn't be necessary (and
jsrrdgraph didn't support it).

This broke graphs rendered by rrdtool with files including a colon in it's
name. Hopefully this is fixed now by double escaping them.

Reported-by: Poil <poil@quake.fr> (https://github.com/pommi/CGP/issues/20)
type/default: urlencode filenames when using canvas

Since commit v0.3-97-g5793a8c [integrate jsrrdgraph in CGP], double quotes have
been removed around filenames and legend because it shouldn't be necessary (and
jsrrdgraph didn't support it).

This broke graphs rendered using jsrrdgraph with files including spaces in it's
name. Hopefully this is fixed now by rawurlencoding them (but the /).