Fix the function signatures of Canvas.toDataURL() and Canvas.toBlob()
and make both functions accept non-numbers as the quality parameter, in
which case it will just use the default quality instead of raising an
exception.
This makes toDataURL.arguments.1.html, toDataURL.arguments.2.html and
toDataURL.jpeg.quality.notnumber.html in
wpt/html/semantics/embedded-content/the-canvas-element pass :^)