Require content-length in POST & Upload requests (#3671)

Avoid passing size = -1 to PutObject API by requiring content-length
header in POST request (as AWS S3 does) and in Upload web handler.
Post handler is modified to completely store multipart file to know
its size before sending it to PutObject().

cmd/typed-errors.go

@@ -33,6 +33,9 @@ var errContentSHA256Mismatch = errors.New("Content checksum SHA256 mismatch")
 // used when we deal with data larger than expected
 var errSizeUnexpected = errors.New("Data size larger than expected")
 
+// used when we deal with data with unknown size
+var errSizeUnspecified = errors.New("Data size is unspecified")
+
 // When upload object size is greater than 5G in a single PUT/POST operation.
 var errDataTooLarge = errors.New("Object size larger than allowed limit")