initialising headers map if it's not there, adding content type header only if it's missing
initialising headers map if it's not there, adding content type header only if it's missing

tjcunliffe/hoverfly,tjcunliffe/hoverfly,SpectoLabs/hoverfly,tjcunliffe/hoverfly,tjcunliffe/hoverfly,tjcunliffe/hoverfly,SpectoLabs/hoverfly,tjcunliffe/hoverfly