Tweak context and fixes #37

[tsileo]

Hey, here is the fix as discussed.

Thanks.

[rgrinberg]

CI needs to be fixed:

[ERROR] The compilation of mustache failed at "/home/opam/.opam/4.07/bin/dune runtest -p mustache -j 71".
- spec_mustache alias lib_test/runtest (exit 2)
- (cd _build/default/lib_test && ./spec_mustache.exe)
- Rendering of test Implicit Iterator - String from sections.json failed.
- Fatal error: exception Mustache_types.Invalid_param("Lookup.scalar: not a scalar")
- test_mustache alias lib_test/runtest (exit 2)
- (cd _build/default/lib_test && ./test_mustache.exe)
- Fatal error: exception Failure("Rendering 0 of test 2 failed: Mustache_types.Invalid_param(\"Lookup.scalar: not a scalar\")")

[tsileo]

I missed the failing CI. I tried to take a look, looks like this "fix" add some regressions.
I don't have enough knowledge/cannot figure out why :(

[gasche]

This tricky issue was fixed by #49, so closing this one. Thanks!