[Reviewer: Alex] Various fixes

Should be uncontroversial - fixes https://github.com/Metaswitch/sprout/issues/538. The corresponding code changes are:

https://github.com/Metaswitch/sprout/issues/700
https://github.com/Metaswitch/sprout/issues/698
https://github.com/Metaswitch/sprout/issues/697
https://github.com/Metaswitch/cpp-common/issues/165
https://github.com/Metaswitch/sprout/issues/695
https://github.com/Metaswitch/sprout/issues/694
https://github.com/Metaswitch/sprout/issues/693
https://github.com/Metaswitch/cpp-common/pull/163
It doesn't feel like the acceptable Accept header values should be in the summary. Also it would be nice to have the actual Accept header in the detail as well.