Now extra_post option of submit method accept also a list of values. Fix some bugs in submit method.
Now extra_post option of submit method accept also a list of values. Fix some bugs in submit method.

lorien/grab,liorvh/grab,SpaceAppsXploration/grab,shaunstanislaus/grab,codevlabs/grab,subeax/grab,DDShadoww/grab,istinspring/grab,DDShadoww/grab,raybuhr/grab,alihalabyah/grab,kevinlondon/grab,raybuhr/grab,pombredanne/grab-1,huiyi1990/grab,shaunstanislaus/grab,maurobaraldi/grab,giserh/grab,liorvh/grab,giserh/grab,SpaceAppsXploration/grab,subeax/grab,codevlabs/grab,pombredanne/grab-1,istinspring/grab,huiyi1990/grab,maurobaraldi/grab,subeax/grab,alihalabyah/grab,lorien/grab,kevinlondon/grab