Proposal for issues #1937 #1938 and #1946

#1937: split HttpPostRequestDecoder in 2 subclasses (Multipart and
Standard) while still backward compatible by using
HttpPostRequestDecoder as before
#1938: if Content-Type is not specified, reverting to standard
application/octet-stream by default (Opera bug)
#1946: allow to set an upper limit on attribute size, for all type of
attributes, including FileUpload

This could be copied to Version 4 too.