because Url.Path already starts with one.
Avoid crashing in Request.ParseForm if there is no body.
R=rsc
APPROVED=rsc
DELTA=5 (4 added, 0 deleted, 1 changed)
OCL=30552
CL=30607
remove io.ErrEOF.
rename io.FullRead to io.ReadFull, to match
ReadAtLeast and ReadAll.
remove io.FullReader, because it is now unused.
R=r
DELTA=295 (88 added, 105 deleted, 102 changed)
OCL=30544
CL=30588