ensure that header exists before accessing it

[swelther]

I use this gem through the savon gem. In my project I noticed a small bug:

If the first (savon) call fails (e.g. because of a typo in the WSDL URL) the next call to the same savon object crashes with this callstack:

undefined method `include?' for nil:NilClass
/home/swelther/.rvm/gems/ruby-2.0.0-p451/gems/httpi-2.4.0/lib/httpi/adapter/net_http.rb:103:in `negotiate_ntlm_auth'
/home/swelther/.rvm/gems/ruby-2.0.0-p451/gems/httpi-2.4.0/lib/httpi/adapter/net_http.rb:80:in `block in do_request'
/home/swelther/.rvm/gems/ruby-2.0.0-p451/gems/httpi-2.4.0/lib/httpi/adapter/net_http.rb:79:in `do_request'
/home/swelther/.rvm/gems/ruby-2.0.0-p451/gems/httpi-2.4.0/lib/httpi/adapter/net_http.rb:34:in `request'
/home/swelther/.rvm/gems/ruby-2.0.0-p451/gems/httpi-2.4.0/lib/httpi.rb:160:in `request'
/home/swelther/.rvm/gems/ruby-2.0.0-p451/gems/httpi-2.4.0/lib/httpi.rb:126:in `get'
/home/swelther/.rvm/gems/ruby-2.0.0-p451/gems/wasabi-3.5.0/lib/wasabi/resolver.rb:43:in `load_from_remote'
/home/swelther/.rvm/gems/ruby-2.0.0-p451/gems/wasabi-3.5.0/lib/wasabi/resolver.rb:33:in `resolve'
/home/swelther/.rvm/gems/ruby-2.0.0-p451/gems/wasabi-3.5.0/lib/wasabi/document.rb:142:in `xml'
/home/swelther/.rvm/gems/ruby-2.0.0-p451/gems/wasabi-3.5.0/lib/wasabi/document.rb:160:in `parse'
/home/swelther/.rvm/gems/ruby-2.0.0-p451/gems/wasabi-3.5.0/lib/wasabi/document.rb:147:in `parser'
/home/swelther/.rvm/gems/ruby-2.0.0-p451/gems/wasabi-3.5.0/lib/wasabi/document.rb:64:in `soap_actions'
/home/swelther/.rvm/gems/ruby-2.0.0-p451/gems/savon-2.11.1/lib/savon/operation.rb:23:in `ensure_exists!'
/home/swelther/.rvm/gems/ruby-2.0.0-p451/gems/savon-2.11.1/lib/savon/operation.rb:15:in `create'
/home/swelther/.rvm/gems/ruby-2.0.0-p451/gems/savon-2.11.1/lib/savon/client.rb:32:in `operation'
/home/swelther/.rvm/gems/ruby-2.0.0-p451/gems/savon-2.11.1/lib/savon/client.rb:36:in `call'

This PR adds a check if the header is really present and resolves my bug from above.

[rogerleite]

Hi @swelther! Thanks for this PR! #148 seems to be the same scenario.

[swelther]

It's worth a try, let's see what the issue creator says :)