ox-latex.el: Downcase language name in minted blocks
* lisp/ox-latex.el (org-latex-export-to-latex): Downcase the language
name when using minted highlighting for src block exports.
Minted, by convention, uses all lowercase names for languages. This
will handle mappings like "R" to "r" or "Python" to "python"
automatically. More complex renamings still need to be handled by
customizing `org-latex-minted-langs'.
Note that the `downcase' is only applied when using the "automatic"
name. Names in `org-latex-minted-langs' still need to be lower-case.
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