Work around sphinx-doc/sphinx#2280
Work around sphinx-doc/sphinx#2280

The docs are currently building with warnings because one HTTP
sourcecode block cannot be highlighted. The root cause is
sphinx-doc/sphinx#2280. Once that is fixed, the block should be fine as
previously written. Until then, explicitly add a Content-Type line to
avoid the bug.

Fixes #41

auvsi-suas/interop,justineaster/interop,auvsi-suas/interop,justineaster/interop,auvsi-suas/interop,auvsi-suas/interop,justineaster/interop,justineaster/interop,justineaster/interop