the x-headers middleware appears to be incompatible with seq header values

Read ring.middleware.x_headers and ring.middleware.default_charset, starting at the stack-trace locations in x_headers.clj and default_charset.clj. Reproduce the failure with a response header whose value is a sequence, then inspect the existing middleware tests. Done means the supported sequence-valued header no longer raises this exception and the behavior is covered by a regression test.

Using a header such as {"content-type" ["text/plain" "text/html"]} in the response will result in the following exception:

[clojure.core$re_matcher invokeStatic core.clj 4667]
  [clojure.core$re_find invokeStatic core.clj 4716]
  [clojure.core$re_find invoke core.clj 4716]
  [ring.middleware.default_charset$text_based_content_type_QMARK_ invokeStatic default_charset.clj 7]
  [ring.middleware.default_charset$text_based_content_type_QMARK_ invoke default_charset.clj 6]
  [ring.middleware.default_charset$add_charset invokeStatic default_charset.clj 15]
  [ring.middleware.default_charset$add_charset invoke default_charset.clj 13]
  [ring.middleware.default_charset$wrap_default_charset$fn__12380 invoke default_charset.clj 27]
  [ring.middleware.not_modified$wrap_not_modified$fn__12337 invoke not_modified.clj 52]
  [ring.middleware.x_headers$wrap_xss_protection$fn__11091 invoke x_headers.clj 71]
  [ring.middleware.x_headers$wrap_frame_options$fn__11079 invoke x_headers.clj 38]
  [ring.middleware.x_headers$wrap_content_type_options$fn__11085 invoke x_headers.clj 53]

Not sure if this is the expected behavior as the spec states that header values can be seqs.