Add setter for Context

In d9259e3 a getter was added
for the context, this adds a setter.

cc @LucasRoesler

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

Add Context

Requested by @spawn2kill in PR #6, however there was no issue
and no instructions for testing. This is being put through in
a separate commit to speed up the feedback cycle given all the
comments and two commits which need to be squashed, this seems
faster.

Closes: #6

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

Adds method to Request struct

Signed-off-by: Edward Wilde <ewilde@gmail.com>