Evaluation of the Immunogenicity of the Synthetic α-Melanocyte-Stimulating Hormone (α-MSH) Analogue Afamelanotide ([Nle-D-Phe(7)]-α-MSH, Scenesse®) in Erythropoietic Protoporphyria Patients by ELISA Detecting Both Anti-Afamelanotide and Anti-α-MSH Antibodies.