if-none-match header causes 500 internal server error on CORS requests

Browsers can send a if-none-match header to couchdb, but this can will break when CORS is enabled. The reason seems to be the header comes in as a binary, eg from the debug log the header is:

{'If-None-Match',"\"V9J3604P4V9VTR5H77SBHJW\""}

,

When maybe_apply_cors_headers is called, the assumption is that the headers are strings. This can be seen here: https://github.com/apache/couchdb/blob/b0420f9006915149e81607615720f32f21c76725/src/couchdb/couch_httpd_cors.erl#L219

Thus a string:to_lower is undefined as can be seen in this stacktrace:

[info] [<0.148.0>] Stacktrace: [{string,to_lower,
[undefined],
[

{file,"string.erl"}

,

{line,468}

]},
{couch_httpd_cors,maybe_apply_cors_headers,2,
[

{file, "/Users/jan/Work/build-couchdb-mac/build-couchdb/git-build/https%3A%2F%2Fgit-wip-us.apache.org%2Frepos%2Fasf%2Fcouchdb.git%3A06c3f351f158f107a8293c7c6c9b2415b942f0e9/src/couchdb/couch_httpd_cors.erl"}

,

{line,219}

]},
{couch_httpd,send_response,4,
[{file,

See original gist https://gist.github.com/ryanramage/5143964#file-gistfile1-txt-L64