Don't provide Content-Type if it is text/plain, for back-compat

Prior to af919ef, sending a "Content-Type" field would result in:
    # Could not create ticket.
    # Content-Type: Unknown field

As af919ef causes all bin/rt requests to send a Content-Type, this makes
it no longer interoperable with RT servers running versions prior to
4.0.14.

Preserve backwards compatibility by not sending the Content-Type field
if it is "text/plain".
Merge branch '4.0/bin-rt-content-type-backcompat' into 4.0.14-releng
Merge branch '4.0.14-releng' into 4.0-trunk