[downloadable] fix filename HTTP header for simple name with space (closes #2535715) Since d74addac92bb, we export simple ascii filename without any encoding in the `filename` parameter of the `Content-Disposition` header. If this name contains space this will fails, the parameter value will be truncated at the space position. (eg. `filename=jungle babar.txt` read as `jungle`) We need to quote the filename to prevent this (eg. `filename="jungle babar.txt"`). Then literal quote and backslash needs to be escaped too. The new escaping is correct according this extensive test case data base: http://greenbytes.de/tech/tc2231/

\.svn
^build$
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\.pyc$
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\.bak$
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\~$
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^doc/book/en/apidoc$
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.*/data/database/.*\.sqlite
.*/data/database/.*\.config
.*/data/database/tmpdb.*
.*/data/ldapdb/.*
^doc/html/
^doc/doctrees/
^doc/book/en/devweb/js_api/