Bug fix release
If you use the header_hash method for setting your headers in middleware and you opted out of a header (via setting the value to false), you would run into an exception as described in #193
NoMethodError:
undefined method `name' for nil:NilClass
# ./lib/secure_headers.rb:63:in `block in header_hash'
# ./lib/secure_headers.rb:54:in `each'
# ./lib/secure_headers.rb:54:in `inject'
# ./lib/secure_headers.rb:54:in `header_hash'