caching does not work properly when keep.source=TRUE

[lq]

When keep.source=TRUE, the expression generated by parse will have variable "srcfile" attribute by default, so the cache directory name generated by digest is essentially random.

A minimal test file:

\documentclass{article}
\begin{document}
<<keep.source=TRUE, cache=TRUE>>=
message("cache failed")
@
\end{document}

[cameronbracken]

Thanks for the fix!