Fix FormPOST max_file_size exceeded bug.

When using FormPOST, if the size of the file being posted exceeds
max_file_size, a HTTP 499 was being returned rather than HTTP 400.

Change-Id: I48c781735c66eccde3deb6f9c3c184aee964a4a5
Merge "Fix FormPOST max_file_size exceeded bug."