Tweak Content-Type headers.

Ensure that we use Content-Type: text/plain for exception stack traces.

Remove a redundant Content-Type: text/html since parse() sets the
Content-Type for us (and parse might need to use the text/plain type
if an exception occurs during parsing).

Change-Id: Ib01a4e7f7f50a92978cf1b2083d57ba5756df095
Merge "Tweak Content-Type headers."