doctests beginning with Sage: are silently ignored

[broune]

This doctest is silently ignored:

Sage: 1+1
45

I.e. no warning is displayed, and doctesting the file it is in produces no failures. This is because I wrote Sage: rather than sage:. I don't now if other capitalizations than "sage" should be accepted, but certainly they should not be silently ignored.

I propose to report an error on "Sage:" or any other capitalization of that in any context where "sage:" would be interpreted as a doctest.