Add mutex for writing to result

[kylecarbs]

Fixes #21.

[kylecarbs]

@tcnksm can we merge this?

[randombangaloreguy]

This fix seems to have been pending for a while. @kylecarbs, could you please merge it at your earliest convenience? Thank you!

[kylecarbs]

@randombangaloreguy only those with write access can!

[randombangaloreguy]

Thanks for pointing that out, @kylecarbs. @tcnksm, since you have write access, could you please take a look and merge the fix when you have a chance? Appreciate your help!

[randombangaloreguy]

I have verified the change on top of v0.2.0.

[josharian]

This causes a bunch of go vet failures. Unfortunately, Result is a value type and can't have a mutex added to it. But a local mutex ought to do? It'd be nice to have a test, too. But this is not my project, I'm just a bystander.

For anyone else finding this, the failures require HTTP/2 to reproduce.