A 12-week, randomised, double-blind, placebo-controlled, three-period, cross-over pilot study comparing the effect of salmeterol/fluticasone propionate, fluticasone propionate and placebo on perpheral blood eosinophils and serum IL-5 in response to allergen challenge in asthma subjects when allergen challenge is administered at 1 hour or 11-12 hours post-dose of the dosing interval