Fix issue with non-String header values.

Non-String header values were causing an exception:

    Read error: #<NoMethodError: undefined method `split' for nil:NilClass>
    /app/vendor/bundle/ruby/2.0.0/gems/puma-2.4.0/lib/puma/server.rb:448:in `each'

This fixes that by converting the value, if possible, to a String. If not possible, the header is not sent.

I tested this change on both v2.4.0 and v2.4.1.