The values of @header are expected to be all strings;
WEBrick::HTTPResponse::[]=(key, val) explicitly converts the second
argument to a string and assigns it to @header hash.
However, there were some points in WEBrick internal code that assigns
non-String to @header.  This change fixes the issues.

The values are checked by `header_value =~ /\r\n/` in check_header.
The type confusion caused conflict with removal of `Object#=~`
[Feature #15231].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65984 b2dd03c8-39d4-4d8f-98ff-823fe69b080e