Fix headers when queued in multi
Fix headers when queued in multi

closes #26 (http://github.com/pauldix/typhoeus/issues/issue/26)
The headers must be set when the request is added to multi otherwise
they are not taken into account.

bitherder/typhoeus,telekomatrix/typhoeus,markschmidt/typhoeus,chazsconi/typhoeus,tinfoil/typhoeus,t3hpr1m3/typhoeus,sportngin/typhoeus,xing/typhoeus,xing/typhoeus,typhoeus/typhoeus,bitgal/typhoeus,chazsconi/typhoeus,markschmidt/typhoeus,PericlesTheo/typhoeus,kkirsche/typhoeus,EddyLuten/typhoeus,bitherder/typhoeus,nju520/typhoeus,pravi/typhoeus,AvnerCohen/typhoeus,wcmonty/typhoeus,nevans/typhoeus,tompscanlan/typhoeus