accept a const value ref in Context::set

[cebtenzzre]

Everything seems to compile fine when this is const. It being non-const makes it hard to pass in a temporary (rvalue).

edit: I intend to sign the CLA but it's going to take me some time to figure out the Google Groups stuff that I need.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

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[cebtenzzre]

Sorry for the delay, it took some reading to understand that I could just sign the individual CLA. That is done now.